WRITTEN DISCUSSION TO PAPER TAKEN AS READ:

4 JULy 1969

by A. M. HOPGOOD

The EditorDear Sir,I read with interest the recent paper on the Lewisian of the Outer Hebrides byCoward, Francis, Graham, Myers & Watson (Proc. Geol. Ass.,1969, SO, 387-408).As one of the areas discussed (Barra) has formed the basis for correlation ofstructural and metamorphic histories of the Lewisian elsewhere (Hopgood, 1964;Bowes, Hopgood & Smart, 1968; Bowes & Hopgood, 1969), it is especiallyinteresting to read some of the conclusions embodied in the paper, in particularthat the existence of metasediments within the Lewisian of Barra can be placedon record. While it would begratifying to have confirmation of an earlier sugges­tion that the gneissesat Bagh nan Clach, Scurrival, are metasediments (Hopgood,1964) and that they can in fact be classified as precisely as 'psamrnites' and'pelites' (Coward and others. 1969, table II, 391), it would be of great value toknow the criteria used to support a sedimentary derivation. It might be regardedas significant that garnets, apparently similar to those from north-west Lewis andreferred to by Coward and others (1969,394)as approaching pure almandine witha small amount of manganese, also occur at Scurrival; these garnets are close toalmandine in composition also and contain approximately 27 per cent FeO andabout 1 per cent MnO.

Coward and others (1969,388 and table I) refer to Laxfordian deformation asfollowing the emplacement of 'minor basic intrusions (=Scourie Dykes)'. Becausethere is more than one phase of Lewisian intrusion in the Hebrides, it isdifficult toclassify the events on Barra, and indeed in many parts of the Hebrides in terms ofa simple two-fold classification such as Scourian and Laxfordian, notwithstandingthe fact that the gneisses of the Outer Hebrides, including those of Barra, havebeen regarded as Laxfordian (Watson, 1965). It is encouraging therefore to notethat the authors, having adopted fold-sequence as a basis for description of thegneisses, have beenable to equate them with the Laxfordian. It would certainlybe of considerable general interest, moreover, to know by what criteria the initialstructural relations of the basic masses to which they refer were elucidated and bywhich they can be so unequivocally correlated with the Scourie Dykes of themainland; certainly petrographical and chemical criteria do not uniquely definethegeological age of any set of tholeiitic minor intrusions. It would also be useful toknow in what way these intrusions are related to the minor intrusions which cross­cut the different fold-phases in Barra. The authors will be aware that there ismore than one phase of minor basic intrusion cutting across the fold-Structuresin the gneisses on Barra. For instance, the main fold of the Scurrival Peninsularefolds second generation isoclinal folds in amphibolite containing xenoliths ofquartzo-feldspathic gneiss (Hopgood, 1964; 1970); and folds of the Scurrivalgeneration are themselves cut by minor basic intrusives (ibid.). Further directevidence of a number of generations of minor intrusives on Barra is clearlyillustrated on the shore at Leenish. These phases of basic igneous activity aresubsequent to the onset of the folding in the Barra gneissesand this folding followsthe emplacement of the basic minor intrusives mentioned by Coward and others(1969, table I) and correlated by them with the Scourie Dykes. The authors shouldnow specify the evidence for their conclusions regarding the age of these earlyintrusives.

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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