264 G. H. Kinahan — On MetamorpMc Rocks. 



scribed. Still, however, his arrangement seems to require modifica- 

 tion, as many of the rocks he has put among his granites seem not 

 to be true granites, but rather granitoid rocks, due to the metamor- 

 phism of igneous rocks. 



In the Memoirs of the Irish branch of the Geological Survey of 

 the United Kingdom (Exs. sheets 95, 105, etc.) it has been pointed 

 out that while some granites are undoubtedly intrusive, others seem 

 to have been formed in the position they now occupy by the extreme 

 metamorphism of both igneous and derivate rocks. In that country 

 the metamorphosed sedimentary rocks form three well marked groups, 

 namely, the ScJiist series, the Gneiss series, and the Oligoclasic granite 

 (Gal way-type granite). As, however, some rocks are more sus- 

 ceptible of change than others, on account of their mineral con- 

 stituents, subordinate beds of gneiss will occur in the Schist series, 

 while beds of schist will be found in the Gneiss series, or even on 

 some occasions in the Oligoclasic granite. As mentioned by Mac- 

 CuUoch, so also in this country, the unaltered rock graduates into 

 schist, the schist into gneiss, and the latter into granite. (See 

 Plate VII. and explanation of same.) 



The argillaceous rocks at the first become glazed or micaceous on 

 the surfaces of the most conspicuous structure of the original rock, 

 changing them into argillite or argillaceous schist, while pyrite and 

 such minerals as chiastolite and phyllite are developed, usually on 

 the surface planes of the stratification, lamination, or cleavage, but 

 often promiscuously through the mass. Moreover, the joint lines 

 are often sealed, or a thin film of rock alongside them hardened. 

 Subsequently the rocks change into mica-schist and the like, and 

 thence through gneiss into granite. Some arenaceous rocks at the 

 first become schist (mica-schist or quartzite), while others, on 

 account of their constituents, are gneissic from the first. The latter, 

 although containing the essential constituents, are not typical gneiss, 

 that is, the quartz, mica, and felspar are not arranged in leaves or 

 plates ; nevertheless, in general there is more or less of an incipient 

 foliation. (Plate VII., Fig. 6.) 



Seemingly, as first pointed out by Sorby, in all schists the foliation 

 is induced by and follows the most conspicuous or marked structure 

 in the original rocks ; let it be cleavage or lamination, whether the 

 latter be parallel, oblique, spheroidal, curled, folded, nodular, con- 

 cretionary or conglomeritic ; except perhaps in some argillites and 

 mica-schist which have a sharply folded or frilled foliation, while a 

 similar lamination is rare in argillaceous rocks. This, however, is 

 not necessarily the case in gneiss, for the Gneiss series consists of two 

 sub-divisions. First, ordinary gneiss, having the foliation following 

 similar laws to those just mentioned, and among which subordinate 

 beds of schist are not uncommon. Second, granitoid gneiss, in 

 which the foliation, with rare exceptions, is in parallel lines, and dips 

 at a high angle, if it is not perpendicular. The strike of the foliation 

 of the latter and of the original stratification of the rocks, appears 

 generally if not alway to coincide, but in no other way does the 

 original structure seem to affect it, as all the other structural pecu- 



