530 Geikie — Metamorphic Origin of Granite. 



present state of our knowledge, we are indnced, like old Sir Eoger, 

 to conclude that niiicli may be said on both, sides. It is obvious, 

 however, that many of the so-called eruptive granites must be re- 

 examined, their igneous character having been assumed at a time 

 when as yet the study of metamorphism had made little progress. 

 No disrespect for the work of the laboratory is implied in the belief 

 that the question of the origin of granite and other allied, rocks will 

 ultimately be solved by the field observer. The labours of the 

 chemist have been invaluable, but experience is ever showing us that 

 the chemical or mineralogicdl composition of a crystalline rock can- 

 not always be taken as a test by which to discover its geological 

 nature. For this purpose it becomes necessary to study the behaviour 

 of the rock with surroimding strata, and to mark how these may be 

 affected by its presence. The composition of the sedimentary de- 

 posits must be carefully noted, in order to prove whether they make 

 the raw material out of which a crystalline rock, such as that with 

 which they may be associated, could have been formed. But even 

 after the composition has been ascertained to be such as, under meta- 

 morphic conditions, would readily give rise to the crystalline mass in 

 question, it will still be necessary to examine carefully the junction 

 of the stratified and amorphous rock ; for if the latter ^e eruptive, 

 its association with any particular strata might, of course, be quite 

 accidental. It must also be remembered that contact with an ig- 

 neous rock has often induced a certain degree of change upon beds of 

 aqueous formation. But this appearance, even when the immediate 

 junction of the two rocks remains concealed, is not likely ever to be 

 taken as a proof that the crystalline rock has resulted from the altera- 

 tion of the stratified dejDOsits. If we are thus to guard against hastily 

 inferring the metamorphic nature of a rock, we must also beware of 

 assuming an igneous character merely from the appearance of veins 

 ramifying from crystalline into granular non-igneous beds. This 

 may in general be an excellent test of eruptive origin, but it cer- 

 tainly cannot always prove that the main mass, from which the veins 

 appear to have come, has been forcibly thrust into its present 

 position. 



As the phenomena connected with the quartzless syenites and 

 granites perhaps evince a less intensity of metamorphic action than 

 the corresponding phenomena of true granite, it will be proper to 

 treat of them first. 



JDiorite and Minette.- — I have elsewhere^ given the results of a 

 detailed examination of the metamorjDhic rocks of Carrick in Ayr- 

 shire, and shown how passages can be traced from granular and 

 comparatively unaltered greywackes into distinctly crystalline and 

 igneous-like rocks, such as felstone, felspar-porphyry, felspathic, 

 amygdaloid, diorite, etc. None of the rocks of that region, however, 

 are micaceous. But in another smaller area of metamorphism, near 

 the little village of Sorn, Ayrshire, a series of felspathic sandstones 

 of Lower Old Red age have become changed in places into a rock of 



1 In a paper read before the Geological Society, June 6, 1866 : — vide Geol. Mag. 

 p. 321. 



