portland stone; its geology and properties as a building stone

225

PORTLAND STONE; ITS GEOLOGY ANDPROPERTIES AS A BUILDING STONE.*

By F. H. EDMUNDS, M.A., and R. ]. SCHAFFER, B.A., B.Sc.

[Received 28th April, 1932.]

[Read 8th April, 1932.]

pORTLAND Stone has been used in so many buildings inLondon that its appearance is familiar to all Londoners.

The oldest known building constructed of Portland Stone isRufus Castle, in the Isle of Portland, built about 1080 A.D.,In the 14th century stone was exported to Exeter in considerable amount and an appreciable quantity was sent to London.It was first used on a large scale in London by Inigo Jones, who,in 1619, employed it for the construction of the Banquetting Hallin Whitehall, and it was extensively used by Wren for St.Paul's Cathedral, and for the numerous other churches which 'herebuilt after the Great Fire. Its use continued during ther Sth and roth centuries, and to-day it is extensively employedover a wide area and enjoys a well-established reputation as adurable limestone which offers a high degree of resistance tothe effects of the smoke-polluted atmospheres of urban districts.

On exposure to the weather, rain-washed surfaces suffererosion, and after a time the shell fragments usually stand outfrom the surface in low relief. In smoke-polluted atmospheressurfaces which are sheltered from rain develop a hard calciumsulphate skin which is usually hidden by a deposit of soot. Thecontrast between the washed and unwashed surfaces gives thestone its familiar black and white appearance. The formationof a sulphate skin, which commonly occurs when limestonesare exposed to town atmospheres, is generally deleterious, becamethe skin tends in time to blister and to flake off. Fortunately,Portland Stone resists this disruptive action better than mostlimestones, but it is not entirely immune. Incipient flakingcan often be observed on old Portland Stone buildings, andadvanced decay was recently found to have occurred beneaththe massive soot deposits which have accumulated under thecornices of St. Paul's Cathedral. Soot is by no means aprotective agent.

While the popularity of Portland Stone is undoubtedly welldeserved, the stone occasionally belies its reputation, and blocksShowing relatively poor weathering qualities can sometimes beobserved in buildings. It is clearly desirable to know in whatway this poor stone differs from the good.

In 1930 a survey of the building stone resources of thecountry was initiated by the Building Research Station and

• Published by permission of the Director of H.M. Geological Survey and the Director of.Building Research. Crown copyright reserved.

F. H. EDMUNDS AND R. J. SCHAFFER, 226

the Geological Survey, and a study of Portland Stone wasselected as the first section of this survey. The investigationwas divided into two parts. An examination of the stone in thefield was undertaken by the Geological Survey, and an examination of the stone in buildings, and laboratory work on samplescollected in the course of the survey were carried out by theBuilding Research Station.

The following account of some of the results of the workis given by permission of the Director of the Geological Surveyand the Director of Building Research. Part I. on field observations is by the first-named author; Part II., by the secondnamed, deals with laboratory work. At this stage it is proposedto give only a general summary. Detailed data will be presented in a departmental memoir which will be published in duecourse.

Acknowledgment is made to Mr. H. J. Sansom for muchhelp and information, and to Mr. H. Smith and Capt. Attwooll,for assistance in the field.

Thanks are due to the Bath and Portland Stone Firms Ltd., toMessrs. F. J. Barnes Ltd., and to the United Stone Firms Ltd.,for facilities to examine the quarries and to collect the samplesused in the investigations described in this paper, and for theunfailing courtesy of their employees.

Mr. Edmunds is indebted to Mr. H. Dewey and to Dr. B.Smith for their helpful suggestions.

The photographs composing Plates 10 and II are taken fromthe Geological Survey collection, and are included by permissionof the Director of the Geological Survey and Museum.

PART I. FIELD GEOLOGY.

Portland Building Stone is obtained from the limestones ofthe Portland Formation, which occurs near the top of the JurassicSeries. The formation is best developed in the Isle of Portland;but rocks of Portlandian age cover a considerable area aroundWeymouth and in the Isle of Purbeck, and they also come tothe surface in the Vale of Wardour, near Swindon, and nearAylesbury. The chief locality providing Portland Stone is theIsle of Portland, the Isle of Purbeck yielding only a smallquantity. Stone is also quarried to some extent in the Vale ofWardour; but is different in type from that from the Dorsetlocalities, and is not here considered.

The geology of Portland has been described by severalauthors, of whom Webster [1]* was the first.

The structural geology of the Island is extremely simple;the whole island forms part of the southern limb of an asymmetrical anticline with an east to west axis, which, in company with

* For references see p. 240.

Geological J1f1p07

PORlL4ND.

FIG. 19.

228 F. H. EDMUNDS AND R. j . SCHAFFER,

the majority of anticlines with east-west axes in the south of England has a steeply-dipping northern limb, and a gently-dippingsouthern limb. The average dip of the st rata is slightly eastof south, at about one and a half degrees, with a maximumrecorded dip of six degrees not ed at Wakeham East quarry.Except for min or undulations, such as that indicated by this localdip at Wakeham, the whole of the beds are entirely free fromfaults and disturbances of any kind; nowhere in th e Island havethe Portland Beds been subjected to exceptional strains andstresses due to earth movements, and consequently the stoneis entirely exempt from shattering and from potential lines ofweakness which are often found in rocks which have undergonesuch strains.

DESCRIPTION OF BEDS.

Purbeck Beds . . Lower P urbeck

Portl andLimest onePortland Beds

The general sequence of beds present is as follows :

Clay and shingleSlatBacon Tier}AishSoft Burr)Dirt Bed fTop CapSkull Cap

[R oachWhit Bed

1Curf and Flinty BedBase Bed RoachBase BedChert Bed s

Portlan d SandsKimmeridge Clay

Portland Beds.PORTLAND SA~DS : Dark green-grey glauconitic sands,

about 125 ft. thick.CHERT BEDS : Buff or white, generally chalky, limestone

with some oolitic grains. Many layers of nodular chert, verysimilar to chalk flints are present. These beds , about 60 ft.thick, yield no stone of economic value.

BASE BED (sometimes called Best Bed): A soft white ooliticlimestone, of fine texture and of uniform colour. Bedding planesare generally absent, and the stone is 'free,' i ,e., it may beworked in any direction. Shells are very rare.

BASE BED ROACH : A shelly oolitic limestone. Thicknessup to 3 ft.; only occasionally present.

CURF: A soft chalky, rarely oolitic limestone. A characteristic of this bed is that no estimate can be made as to thedirection in which a block will split on being struck with a

PORTLAND STONE: ITS GEOLOGY AND PROPERTIES, ETC. 229

hammer. When hit, • stringers' or cracks appear in alldirections. Thickness, zero to 4 ft.

FLINTY BED: A chert bed up to 2 ft. thick and oftenassociated with curf. The two beds together show a recurrenceof conditions similar to those in which the Chert Beds weredeposited.

WHIT BED: A buff-coloured oolitic limestone with a largequantity of comminuted shells; when the' quarry water' hasdried out the stone becomes considerably lighter in colour. Asin the case of the Base Bed, bedding planes are absent and thestone is a freestone.

ROACH: A tough grey-brown oolitic limestone riddled withcasts of shells, mainly Trigonia gibbosa (locally called" horsesheads ") and" Cerithium" portlamdicum (the' Portland Screw').It is interesting to note that Fitton calls this bed" Roche" [2J.

Purbeck Beds.

SKULL CAP: A massive grey-brown tufaceous limestone,showing an abrupt change from the underlying Roach. Thickness variable, from It ft. to 5 ft.

Top CAP: Similar to the Skull Cap. Thickness variesfrom 5 ft. to 15 ft. It is generally reckoned that the two' Caps'together give a thickness of 15 ft.; they consist of a freshwaterlimestone, possibly of algal origin [5J, and are non-fossiliferousas far as animal remains are concerned. Much of the tufaceousdevelopment is frequently called" grizzle," or " grizzly beds,"on account of its appearance. Cavities or " chaff holes," oftencontaining silicified wood, are common in the Caps.

DIRT BED, OR BLACK DIRT: A fossil soil 3 in. to 18 in. thick,containing stones derived from underlying rocks. This bed isconstant throughout the Island; locally, similar beds aredeveloped between the Portland Beds and the Skull Cap, andbetween the Skull Cap and the Top Cap.

SOFT BURR: A calcareo-siliceous rock, about I ft. thick.This bed and the Black Dirt, together frequently contain fossilisedstools of cycads, locally termed" birds' nests," and silicifiedtrunks of coniferous trees (Araucarites). It is remarked by thequarrymen that as a general ru1e these tree-trunks lie in onedirection, with bases towards the west and tops towards theeast. The Soft Burr, like the Caps, is possibly of algal origin.The name is derived from the presence of burrs, or protuberances,formed by the fossil cycads.

AISH: A soft earthy limestone.BACON TIER: A similar bed to Aish, often stained red. These

two beds are generally associated as' Aish and Bacon Tier,' andhave a total thickness of 4 ft. to 8 ft.

230 F. H. EDMUNDS AND R. J. SCHAFFER,

SLAT, OR HARD SLAT: A hard grey fissile limestone. Severalbeds occur in some localities, with seams of unctuous blue claybetween them; the average thickness of an individual bed isabout 2 ft. The rock has been used extensively in Portlandfor dry-walling. It is so named on account of its fissile character.

SHINGLE: A soft, light grey, very fissile limestone, up to5 ft. thick. The name, as in the case of Slat, is associated withthe very fissile nature of the rock.

Slat, shingle and beds of blue clay are collectively termed.<Top Rubble," and several beds of the same lithological typewithin the ,. Top Rubble" group may occur in the same quarry.

Building-Stone Beds.

As has been remarked above, the Whit and Base Bedsprovide the building stone; it is, however, frequently difficultin practice to differentiate between the strata. Where the Curtand the Flinty Bed are present, the division is obvious, butthese beds, together with the Base Bed, are impersistent, andin parts of the Island all three are absent. Further, greatvariability characterises stone on the same horizon in the samequarry. This is recorded by Gray, in the only paper on Portlandread before the Geologists' Association [3J, who says, .. Thequality of the Whit Bed-like all the other beds in Portlandvaries considerably; for example, in one part of the AdmiraltyQuarries it is exceedingly rough and frothy, containing numerousshells and white spots of a calcareous substance . . .whereas, in the same quarry, nay, even at the other side of ajoint or parting, the stone assumes its usual fine and uniformtexture." In the quarries, the line of demarcation, on accountof variability of the beds, between Whit and Base Beds isfrequently arbitrary, and may depend on slight changes of shellycontent, on a bedding plane, or on the personal element ininterpretation. For example, Base Bed roach is frequentlyreplaced by good stone; if Curf is absent, such stone may becalled" Top Tier of Base Bed" or " Bottom Tier of Whit Bed."The position may be summed up by stating that while the BaseBed is at the bottom of the building stone beds and theWhit Bed is at the top, the terms are more applicable tolithological types than to actual horizons: this is indicated byFig. 20, showing quarry sections as recorded by quarrymennear Easton, all within an area measuring one mile by threequarters of a mile. It will be seen that the average thicknessof building stone is about 20 ft.

The Whit Bed is considered at Portland to be by far a betterbuilding stone than the Base (or Best) bed. Gray [3J remarks," The local term .. Best Bed," as applied to the stratum nowunder consideration, has caused no little confusion and dis-


appointment; for, though it possesses the finest texture andthe most uniform colour of any bed on the Island, it is not reallythe best for many of the purposes to which it is at present applied;it is liable to rapid decay when exposed to the weather; .There can be no objection to its use for inside work,but, for outside work it is ruinous." Of the Whit Bed he remarks:"Next above the curf is the Whit-Bed, or the true Best-bedof Portland stone. The local term Whit-bed is a misnomer,

for Whit-bed, in contradistinction to Best-bed,implies that the former is whiter and second in quality to thelatter, whereas, in reality, the Whit-bed is the darkest and best,and (what is called) the Best-bed is the lightest and worst."

"""-rlP>'"

.,.,- "".,., /-,..,"J .,~",,/..,./ ',.,.,

-'.1 .",','/I.

-- --

/ ..,./","/'.,-J

':/.

Fig. 20.-COMPARATIVE SECTIONS SHOWING LITHOLOGICAL VARIATIONS INPORTLAND LIMESTONE.

At the present time Base Bed stone is used in considerablequantities on account of its appearance. Incidentally, thequarry owners at Portland are frequently placed in somedifficulty by orders of "Whit Bed without shells." The maindistinguishing character in a hand-specimen between Whit Bedand Base Bed is the shelly content of the former, and to obtain"Whit Bed without shells" is almost an impossibility.

Several types of minor lithological variation occur; someof these appear to have no effect on the quality of the stone,others are definitely detrimental, and stones affected are rejectedat the quarry. The commonest type of variation is that of the, Mappy Stone.' This stone, when found, occurs just below the


Roach. Apparently it is of the same constitution as the WhitBed generally, but it weathers into a black and white patternthought to resemble maps. This stone is often of first quality,and blocks are incorporated in many London buildings ; onegood example may be seen in the new Devonshire House,about 4 ft. above ground level.

In some quarries, stone is found with included pellets ofchalky material up to I inch in diameter ; this chalky materialis similar to that of the Curf, and occurs in beds immediatelyabove the Curf. This stone is called " Mottled Stone." Althoughotherwise apparently sound, it is of little value on account ofthese pellets, and is not used.

Occasionally irregular veins of calcite, possibly representingcontemporaneous slip in the unconsolidated oolite, occur in abed; these streaks may be a foot or two long, and up to halfan inch wide; they are called" snail-creeps." A snail-creepoccasionally spoils a block of stone, since it is the section of avein harder than the main mass of stone, and blocks tend tosplit along the surface of this vein .

In any part of the building stone beds nodules of chert orflint are liable to occur; these are" sand-holes." A somewhatsimilar flaw is due to a small lens, about 2 in. or 3 in. in diameter,of black material ,probablycoprolitic in origin, termed a" witcheschimney."

Jointing.

The whole of the limestone beds between the Portland Sandsand the Black Dirt are affected by a system of joints on a roughlyrectangular plan, master joints running from north-north-eastto south-south-west, crossed by others, approximately from eastto west. The N.N.E.-S.S.W. joints are usually enlarged,frequently being 2 ft. or more wide, and are termed" gullies " ;they occur at intervals of 60 ft. to 70 It ., and the sides are usuallycovered with stalagrnitic material ; other joints following thistrend, but differing from gullies in being only a few inches wide,are termed " southers," The east-west joints, generally only afew inches wide, are called" east-westers." Other joints are.. nor'<easters," and occasionally a .. ranger" joint is encountered.

Beds above the caps are not jointed to any degree; but areaffected by the joint-system in that their constituent materialsags into the gullies, producing over each gully a zone of brokenand collapsed strata (6].

The origin of the joint-system appears to be tectonic; thegreat mass of limestone has cracked in response to differentialstresses caused by movement in its foundation of the PortlandSands. The width of the gully joints in relation to the east-

PHCC. GEOL. Assoc., VOL. XLIII. (ICJ32). PLATE 10•

.'c. COAST SCE);ERY. EAST SIDE OF 1'0RTLA);D.

B. COAST SCE);ERY, \YEST SIDE OF l'ORTLA);D.

To [ace 1'. 232.


westers may be due to the fact that the beds form part of alarge east-to-west anticline. During the formation of this foldthe tendency may have been for the beds of limestone to becompressed, and even to slip forwards on the Portland Sandsin such a way that east to west cracks would, when formed, beprevented from opening to any great extent. No tectonicpressure was exerted in an E.-W. direction (in fact, there mayhave been turning) and gullies appear to be cracks widened byslipping slightly on the Portland Sands. That the wideningis due to actual separation of blocks in space appears to besubstantiated by the zone of broken material above each gully.Some part of the widening, however, may be due partly tosolution.

This jointing system has a great effect in moulding the shapeof the Island and on quarrying operations.

LANDSLIPS.

Landslips are of fairly common occurrence in Portland, andare worthy of attention. The usual form of landslips is one inwhich the foot of a mass slips outward and upward, so that themass tends to slide away on its back (so to speak) from theparent block. The landslips at Portland, however, are thereverse of this, in that the slipped masses topple towards thesea (Plate IOA). An explanation given by Damon is that theyare due to rain-water collecting in fissures at the top of theKimmeridge Clay [4J. A more likely explanation, involvingthe jointing system, is as follows :-The Portland Sands cropout in the cliffs from about a third the way from the Bill up theIsland to the north; the sea removes large quantities of this bed,which thereby loses much of its buttressing strength. ThePortland Limestones, as stated above, consist of a series ofblocks, separated by joints, particularly by gullies. Blocks,near the coast, by their immense weight, gradually squeezeout the underlying soft bed, thereby losing their level foundations. They tilt seawards, leaving a chasm along the line of thegully-joint originally dividing the slipped block from the mainmass of limestone.

COAST EROSION.

The shape of Portland is an apparent anomaly in that sincesoft Kimmeridge Clay is at sea level on the north of the Islandand hard limestone at the south, marine erosion may be expectedto be more severe on the north, and consequently the Islandshould be wider at the south than at the north; yet the converseis the case. In considering the shape, however, the jointingsystem of the rocks and the tidal currents have to be taken intoaccount as well as lithology.PROC. GEOL. Assoc., VOL. XLIII., PART 3. 1932. 16


It has been remarked above that Portland forms part of thesouthern limb of an asymmetric east-to-west anticline. TheGeological Maps of Dorset [8] show the whole area to be affectedby several such folds arranged en echelon, anticlines passingeastwards into synclines, and an estimate as to the formerextent of the anticlinal limb of which Portland is the last traceabove sea level may be made by studying the distribution of thesefolds; the westerly limit appears to have been southward ofSwyre, and the easterly southward of Poxwell. The originalsouthern limb of the fold at Portland thus comprised a roughlyrectangular block of very gently-dipping beds about 14 mileswide in an E. to W. direction.

A study of the tides [7] shows that the western side of theIsland is washed by a strong southerly current for about eighteenhours out of twenty-four, while the eastern side is so washedfor only about six hours.

Assuming the general trend of currents to have been constantfor a long period of time, this would cause more rapid erosionon the western part of a land mass than on the eastern, and anyresidual land would be located towards the eastern side of theoriginal block. In such a postion is Portland situated; it isabout eight miles east of the longitude of Swyre, and two mileswest of the longitude of Poxwell.

The scour of the tide may be considered to shave off materialfrom the whole length of the western side of the Island, andthe influence of lithology is here seen. It may well be that thenorthern part of the Portland land mass with clay at sea levelwas at one time west of the longitude of the southern part, buthas receded eastward more rapidly, and to-day is eastward ofthe longitude of the Bill. On the eastern side joint planes exerta dominating influence. It has been remarked above thatgully joints run fran N.N.E. to S.S.W., and further, that landslips occur by separation of blocks from the main mass of stonealong these' gullies'; that wastage of the coastal area on thisside of Portland has proceeded in this manner may be seenfrom the amount of slip debris present northward of ChurchOpe. The outline of the eastern side of Portland coincideswith, and is dependent on, the direction of the gully joints. Theeastern side of the original land mass may have been parallelwith the present eastern side.

METHODS OF QUARRYING.

Geological factors have exerted a dominating influence onmethods of quarrying. There is little doubt that the earliestquarries were opened in landslipped masses; since these masseshad a seaward slope it was possible to work from the back of

PHOC. GEOL. Assoc., VOL. XLI II. (1932). PU.TE II.

A . C LIF F S ECTIOX S H OWI NG J'O RTLA XD B UILDI X G STOXE DEDS.T he ru in s of R ufu s Cas t le a t t op . The uppermost bed is " Cap ," wh ich is

underl a in b y b u ild in g sto ne bed s ; these ha ve been quarried a wa y ove r most ofth e a rea a ro u nd t he Castle. Chert beels form the low est st ratu m sho wn.

B . T H E Sl:C K Tl It: \ 1B G RO U P OF Qt:AR RlES.

The scale of these q uarries is sho wn by the figu res o f t hr ee m en with inthe circle .

:T D f ace p. 13-1 .


the slip without removing the overlying 'cap.' It is highlyprobable that much of the stone used in St. Paul's Cathedralcame from slipped masses; in fact, the lifting apparatus on theIsland at that time was insufficient to work much of the stonefrom undisturbed areas; this explains an Act of 1669, passedat Wren's request, prohibiting the export of stone from Portlandin case there should be insufficient for the requirements of St.Paul's. It is thought that hand-jacks were the only liftingapparatus available at that time; crab winches and shear legswere used until about 1860, when the first crane, a hand-cranewith a chain and with a fixed gib, was introduced. Laterdevelopments in lifting apparatus have been hand-cranes withmovable gibs and with wire ropes, followed by steam cranescapable of lifting blocks up to 15 tons, again followed, in 1931,by electric cranes.

In present-day quarrying the' Top Rubble' of the PurbeckBeds is removed; the effect of the gully joints on these beds hasbeen indicated. In quarrying, operations are carried on fromgully to gully, and the sagging in the Purbecks is of use to indicateto quarrymen where a gully occurs lower down. On reaching the< Cap' this bed is blasted and removed. The roach, andsucceeding blocks of building stone, are quarried in the followingmanner :-Vertical and horizontal grooves, termed" pits," arecut, which are lined with steel plates, or " scales" and a seriesof wedges, sometimes up to 30 in number, placed side by side,are inserted, and then hammered; the stone splits cleanlyand is hoisted out of the quarry, being trimmed square bythe same method of scales and wedges, after which it is" scappled " or trimmed; any flaw present betrays itself underthis treatment by the stone splitting along any weak line.

The horizontal plane along which a block splits off is termeda "rising." As remarked above, bedding planes are generallyabsent in the building stone, but any suggestion of a beddingplane, such as a thin bed of shells lying horizontally, is utilised:on account of their use in quarrying operations, bedding planesand junctions between beds are referred to as "risings."

One of the great difficulties of Portland quarrying is thedisposal of the rubble and cap. In some quarries the overburden amounts to 60 ft., and this refuse is built into massive-" beaches," almost up to the quarry face. This method ofquarryin~ appears to be the most economic one in Portland.Mining is not practicable, for a variety of reasons; e.g., there is noset place along which stone would split to give a roof; theproximity of the gullies to each other also precludes a good roof:no blocks of large size could be produced, and pillars of stone forsupport would have to be left to such an extent that great wastewould occur.


QUARRY MARKS.

Every block of stone is given several quarry marks; anexample of these is that of Wren's quarrymen, which was placedon all stones passed for shipment to London; this mark islocally known as the" wineglass mark," and is shaped somewhatlike a Y, with a bar across the top. Some of Wren's stones,collected near the East Weares, are still to be seen, and occasionally a block is found bearing this mark. All marks wereformerly cut with a sharp axe, and consequently curves were

TAlLY SYMBOLS used in QlIARRIES

20

PORTLANIJCubicFeetSymbol

III - 1111V

VI - Villi

Symbol

*50 -f+HtHalf8hownbyaTick.

+ Jh V+1 - tllll J9J2 :mtV#fftort! Above j9~arabic1zI- _ numerals used.

CubicFeet

;5-45

6-910

n-1415

, 16-19FIG. 2I.

absent. As the stone is " free," quarry marks could be cut inany direction.

Among other marks is one showing the cubic measurement ofthe stone in feet, and in this connection an interesting survivalof old tally marks is utilised (fig. 21). These symbols, theorigin of which is lost in antiquity, are eminently suitable forbeing cut with an axe, but at the present day they are madewith paint. They show traces of Roman influence, but may inpart be even earlier.


PART II. LABORATORY INVESTIGATIONS.The samples collected in the course of the survey, together

with four samples of weathered Portland Stone of known quality,which were obtained from various buildings, were submitted tolaboratory examination.

The problem of the weathering of building stones has beenunder investigation at the Bnilding Research Station for someyears, and it was already known before these tests were undertaken that the reason for the variation in quality of Portlandand other limestones is to be sought in differences in physicalstructure rather than in differences in chemical composition.The experimental work followed the lines which had been foundby experience to be of practical utility in the study of theproblem.

Crystallisation Tests.The first stage in the investigation was to subject samples to

a crystallisation test involving the crystallisation of sodiumsulphate in the pores. Specimens of standard size were soakedin a I4 per cent. solution of sodium sulphate deca-hydrate andwere dried in an air oven under rigidly controlled conditionswhich have been adopted by the Building Research Stationfor standard tests. The process was repeated fifteentimes, the specimens being weighed after each cycle ofoperations.

The use of this crystallisation test for the purpose of estimating weathering quality is, admittedly, a somewhatarbitrary procedure, because sodium sulphate, though responsiblefor much decay in building stones, is not the predominantcause. Nevertheless, experience of its use had already demonstrated that its effects accord with those brought about byordinary weathering conditions [9J, and the results of the presenttests have served to strengthen that opinion. Crystallisationtests appear to have originated with Brard [10J, who attemptedby this means to provide a method of estimating frost resistance,but it is to be clearly understood that the test is hereregarded as a weathering test and not as an artificial freezingtest.

The effect of the test on the Portland Stone specimens variedconsiderably. Some were not appreciably affected; otherswere seriously disintegrated. In those instances where thesamples had been obtained from buildings the agreement betweentheir behaviour under the test and under natural weatheringconditions was extremely good. For example, a specimen ofsound stone taken from the ruined church of St. Andrew on thecoast of the Isle of Portland was practically unaffected; aspecimen from a block of stone which had suffered unusuallyrapid decay in a building in London failed almost completely;

F. H. EDMUNDS AND R. J. SCHAFFER,

and two specimens from St. Paul's Cathedral, one of betterquality than the other, fell into appropriate positions in thetable of results. Moreover, three of four samples which weredescribed as being from the Curf Bed, which is known to be ofpoor durability, failed under the test. The fourth proved tohave a structure similar to that of stone from the Whit andBase beds, and was, in fact, from a quarry where the Cud Bedis but poorly developed.

This evidence, considered in conjunction with the resultsof the test on other building stones, is thought to provide anadequate basis for the opinion that the crystallisation testserves to provide a reliable indication of weathering quality.

The next step was to ascertain what structural characteristicsdetermine these differences in behaviour.

Microscopical Examination.

Portland Stone is a fine-grained, oolitic limestone, the averagediameter of the grains being approximately 0.3 mm. Acharacteristic feature of its structure is that the oolitic grains,which are themselves porous, are united at their points ofcontact, but the spaces between the grains form large, interconnected pores; inter-granular calcite occurs only sporadically(Plate 12). In this respect it differs essentially from ooliticlimestones of the Bath Stone type in which the grains are set ina continuous matrix of calcite. The pore space in PortlandStone can, therefore, be divided into two types, viz., the largeintergranular pores, and the small pores within the grains themselves. For convenience of description these are described as" macropores" and" micropores" respectively. This characteristic structure obtains in the Roach, Whit and Base beds,there being little structural difference between samples frcmthese beds except in the proportion of shell fragments or shellcasts. The stone from the Cud Bed is seldom oolitic, but hasan indefinite, chalky type of structure.

Excluding the Curf Bed, the only obvious variation instructure which might account for the variation in behaviourunder weathering conditions or under the crystallisation testwas to be found in the porosity of the oolitic grains. Microscopical examination showed that samples which exhibited thehighest degree of durability had dense, non-porous grains;those which were less durable had relatively porous grains.Quantitative measurements of macro- and microporosity couldhave been made with the aid of the microscope, but such measurements are tedious and time consuming. Recourse was thereforemade to a capillary method, which was developed for the purpose, by means of which the requisite information can beobtained with comparative ease.

PIWC. GEOL. Assoc., VOL. XLIII. (1932). Pr.YrE 12.

l'ORTLA"D STO:O;E-·· ·\YHIT BED (HIGHER HEADLAKDS (jUARRV).

l\.B.-.-'\ d vcd synthetic resin \yas usee! to fill the pores prior to sectioning. Under the microscope the resin is clearly distinguishable byits colour. In the photomicrographs (x 30 and x (io) it can be distinguished from intergranular calcite by its mottled appearance, but itspresence in the micropores is not clearly revealed.

[To fare p, 238.


Capillary Measurement of Microporosity.A measure of the capillary suction of a porous material

can be obtained by a " capillary" rise method [11J in which thesuction force, developed by the evaporation of water from thesurface of the saturated material, is made to cause a rise in thelevel of a column of mercury placed in hydrostatic connectionwith the water contained in the pores. But the measure soobtained is the capillary suction of the nearly saturated materialand represents the suction of the larger pores. The capillarysuction of the small pores cannot be determined by this method.Most samples of Portland Stone tested under these conditions give a value equivalent to a head of about 200 ems. ofwater.

In order to measure the capillary suction at moisture contentsless than saturation the method has been modified bv Messrs.B. H. Wilsdon and L. F. Cooling of the Building Research Stationin a manner which is described in detail elsewhere [12]. Itconsists essentially in allowing weighed samples to corne toequilibrium with a plate of fine-pored material arranged in asuitable apparatus in such a way that a measured suction force,or negative hydrostatic pressure, can be exerted on the watercontained in its capillaries. At equilibrium only those poreswhich are capable of exerting a capillary suction equal to orgreater than the imposed suction are filled with water. Byrepeating the measurements at various negative pressures thepore structure can thus be explored quantitatively.

Equilibrium may be approached either from the wet or fromthe dry condition. At equilibrium the moisture content is foundto be somewhat higher when approached from the wet statethan when the specimen is initially dry. This hysteresis effectis probably due to differences in the angle of contact betweenthe liquid and the solid under wetting or drying conditions, andalso, to some extent, on the shape and relative distribution ofthe pores.

Preliminary tests on a selection of samples of Portland Stoneshowed that the macropores exert a suction equivalent toa hydrostatic head of about zoo ems. of water (correspondingwith the value obtained by the capillary rise method) and themicropores a suction equivalent to a head of more than 700 ems.There was relatively little change in moisture content atintermediate values. To provide a measure of macro- andmicroporosity,systematic tests were therefore made at a negativehydrostatic pressure of 600 ems. of water. This correspondstheoretically to the capillary suction exerted by a circular poreof diameter 0.005 mm. For the present tests equilibrium wasapproached from the saturated condition, the samples beingsaturated in vacuo. At equilibrium the amount of water lost

240 PORTLAND STO NE: ITS GEOLOGY AND PROPERTIES, ETC.

is a measure of the space occupied by macropores; thatremaining, a measure of the space occupied by micropores.

The results of these experiments served to confirm the microscopical observation that, in general , the quality of a sample ofPortland Stone, as evidenced either by the effect of naturalexposure or by the crystallisation test , depends on its microporosity , and it was found that this conclusion applies notonly to the oolitic type s, but also to the samples from the CurlBed. Similar experimen ts on a further set of samples collectedfrom one particular group of quarries have provided confirmatoryevidence.

Of these latter samples, those from the Whit Bed appear tobe of somewhat better quality than those from the Base Bed,but the results on the first series of tests, that is , on samplesdrawn from the whole area, show that this is not always so.Samples from the Roach Bed have shown variations in charactersimilar to those observed with samples from the Whit and BaseBeds.

It is concluded from the laboratory examination that thedurability of a sample of Portland Stone is largely controlled bythe porosit y of the oolitic grains, and that a reasonably reliableestimate of weathering qu ality can be obtained by observationof behaviour under th e crysta llisat ion test , and measurement ofthe microporosit y .

Further aspect s of th e problem are under investigation.

REFERENCES.1. 'VEBSTER, T. 1829. T rans. Geol, Soc., znd seri es, vol , ii., p. 41.2 . FITTON, W. H . 1836. Tr ans. Geol, Soc., znd series, vol. iv., p. 21g.3 . GRAY, W . 1859-1865 . Proc. Geol, A ssoc., vol. i., p . 128 (Paper

read 1861).4. DA~ION, R. 1884. Geo logy of W eymouth, znd ed .5. STRAHAN, A. 1895. The Jurassic Rocks of Britain (Mem. Geol,

SlIrv.l, vol. v .1898 . The Geolo gy of the I sle of Purbeck a nd Weymouth (Mem.

Geol, Suyv. ), pp. II 2- I21.6. FISHER, O. 1863 . Geologist , vol. vi., p. 250.7. Channel Pilot, 1931. P art I. Hydrographical Dept., Admiralty,

r zth edition, p. ISS.8, One-inch Old Series Geological Map, Sheet 17.

One-inch New Series Geological Maps, Sheets 328, 329, 342, 343.9. SCHAFFER, R. ] . 1932 . The Weathering of Natural Building

Stones. London: H.I\L Stationery Office.10 . BRARD, - . 1828 . On the method proposed by Mr . Brard for the

immediate detection of stones unable to resist the action offrost. H eri cart de Thury : Annales de Chemie et de Physique,vol. 38, 160-92 .

II. Building Re search Board. 1929. Report for the year 1928, page12. London : H .M. Stationery Office.

:12. 1932. Report for the year 1931, page 21. London:H.l\1. Stationery Office .

portland stone; its geology and properties as a building stone

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