reply by m. p. coward, p. w. francis, r. h. graham, j. s. myers and j. watson

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REFERENCESBOWES, D. R., A. M. HOPGOOD & J. SMART. 1968. Glasgow University Exploration

Society Expedition to Inishtrahull. Nature, Lond., 217, 344.--- & - ---. 1969. The Lewisian Gneiss Complex of Mingulay, Outer

Hebrides, Scotland. Geol, Soc. Amer. Mem., 115, 317-60.COWARD, M. P., P. W. FRANCIS, R. H. GRAHAM, J. S. MYERS & J. WATSON. 1969.

Remnants of an Early Metasedimentary Assemblage in the Lewisian Complexof the Outer Hebridges, Proc. Geol, Ass., 80, 387--408.

HOPGOOD, A. M. 1964. Structure and Tectonic History of Lewisian Gneiss, Isle ofBarra. Ph.D. Thesis, Univ, St. Andrews.

----. 1970. Structure and Tectonic History of Lewisian Gneiss, Isle of Barra,Scotland. Krystalinikum, 7.

WATSON, J. 1965. Lewisian. In G. Y. Craig (Editor): The Geology oj Scotland, 50--77.Edinburgh.

A. M. HOPGOOD

University of St. AndrewsDepartment of GeologySt. AndrewsFife, Scotland

REPLY BY M. P. COWARD, P. W. fRANCIS, R. H. GRAHAM, J. S. MYERS AND J. WATSON

The Editor

Dear Sir,

Dr. Hopgood appears to be raising three principal queries in his most welcomecontribution:

First, the criteria used to support a sedimentary origin for the metasedimentsat Seurrival, Barra.

Second, the grounds for correlating the basic masses in the Hebrides with thoseon the mainland, the Scourie dykes.

Third, the relationships between these basic intrusives and the other suites ofminor intrusives on Barra.

The 'metasediments' at Scurrival Point were distinguished as lithologicaltypes by Jehu and Craig as well as by Dr. Hopgood himself. They are by no meansoutstanding examples of their kind, and if considered in isolation, wouldprobably not be so called. They so strongly resemble, however, the moreextensive and more convincing assemblages found elsewhere in the Hebrides,that a similar derivation seems not unlikely. It was observed in the paper,however, that some supposed 'metasediments' may be unusual in that they owetheir existence or preservation to the presence of nearby large basic bodies. TheScurrival rocks clearly fall into this category.

Secondly, the correlation of the suite of basic intrusives in the Outer Hebrideswith the Scourie Dyke suite of the mainland was first made by Dearn1ey (1962)and this link was supported by his later work at Ardivacher Point (Dearnley &Dunning, 1968). Dr. Hopgood is referred to these publications for furtherdetails. Our work has abundantly confirmed Dearnley's correlation, and inparticular age dates obtained recently from Barra and other parts of the OuterHebrides provide further convincing evidence to support our interpretations ofHebridean geology in terms of Scourian and Laxfordian (see forthcoming papersby Francis & Moorbath and Coward and others).

Third, the basic intrusives correlated with the Scourie dykes clearly post-datethe other minor intrusive suites of intermediate and basic rocks recognised on

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Barra. No evidence was found by us for any fold phases separating intrusiveevents represented by these suites, and it is our view that the only deformation toaffect the dyke suites on Barra wasLaxfordian. We cannot agree that 'folds of theScurrival generation are themselves cut by minor basic intrusives'.

Department 0/ GeologyImperial Col/egePrince Consort RoadLondon S. W.7