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V.—Metamorphic Rocks of Scotland and Galway
Published online by Cambridge University Press: 01 May 2009
Extract
It appears evident from the history of the metamorphic, granitoid, and granitic rocks of Scotland, written exactly halfa century ago by MacCulloch, that those rocks are very similar to rocks of the same classes in West Galway, Ireland. This acute observer evidently examined the Scotch rocks most minutely, as the groups, sub-groups, and varieties of his “primary rocks” are carefully classed and described. Still, however, his arrangement seems to require modification, as many of the rocks he has put among his granites seem not to be true granites, but rather granitoid rocks, due to the metamorphism of igneous rocks.
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References
page 263 note 1 A Geological Classification of Rocks, etc., by John MaeCulloeh, M.D., F.K.S., etc. A.O. 1821.
page 266 note 1 In the description of the Geology of Abyssinia (Geology and Zoology of Abysginia, by W. T. Blanford, p. 169), the learned author calls particular attention to the regular and nearly perpendicular foliation of the gneissic rocks of both that country and also the southern part of the Indian peninsula. This is evidently similar to the foliation in the granitoid-gneiss of West Galway, and probably is due to the same cause. In the countries to which he refers, Blanford suggests that cleavage induced the foliation; this, however, could not be the case in West Galway, as evidently cleavage was not developed in the associated metamorphosed sedimentary rocks. Therefore, it is impossible it could have existed in tbe portions now changed into granitoid-gneiss, more especially as some are small and isolated, and may surround a centre that has been changed into typical granite. To me, therefore, it seems that this linear-parallel, nearly perpendicular, foliation of granitoid-gneiss must be due to a high state of metamorphism, and not to an original rock-structure; and if this has been the cause in West Galway, it was also probably the cause in Abyssinia and India.
page 267 note 1 MacCulloch, p. 234.
page 267 note 2 MacCulloch, p. 235.
page 267 note 3 MacCulloch, p. 236.
page 268 note 1 MacCulloch, p. 237.
page 268 note 2 MacCulloch, p. 238.
page 268 note 3 MacCulloch, p. 241.