A Composite Gneiss near Barna (County Galway)

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Abstract
The great area of granite, probably of pre-Cambrian age, stretching west from Galway town, includes a variety of rock-types, some of which are of composite origin. Gneisses (foliated granites) occur along the northern margin, the foliation, marked out by their dark constituents, being parallel with the bedding of the adjacent metamorphosed sediments. Dr. C. Callaway observed in 1887, near Glendalough (Recess), phenomena similar to those which have since formed the basis of the ‘stoping’ theory of intrusion, and he noted how ‘the granite has intruded between the strips of diorite and pushed them apart, without materially disturbing their parallelism.’ He records the coincidence of the foliation in the Galway granite with that of the enclosing schists, and attributes this to pressure acting during the consolidation of the mass. It is remarkable that he did not take the further step of pointing out how the strike of the sedimentary series has influenced that of the foliation in the igneous invader. Composite rocks, where diorite has become included and streaked out in the granite, occur in the west of Townparks townland, in the demesne of St. Mary's College in Galway town. Some of these are referred to in the Memoir of the Geological Survey of Ireland [1869] accompanying Sheets 105 & 114, where Kinahan (p, 25) treats the whole as a metamorphosed sedimentary series. Prof. Watts describes some of the diorites as ‘doubtless intrusive into the granitite,’ a conclusion that does not seem to be sustained by the field-evidence. Kinahan ( op. cit.

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