266 G. -H". Kinahan — On Metamorphic Rocks. 



liarities are obliterated, save and except when the original rock was 

 congiomeritic, some blocks in which may have resisted the action 

 which tends to make the foliation regular and nearly perpendicular ;^ 

 consequently in some granitoid gneiss, blocks or nodules may occur 

 differing in appearance, structure and composition from the rest of 

 the mass ; moreover they nearly always, more or less, deflect the 

 strike of the foliation in their vicinity ; but they are generally more 

 or less elongated in the direction of the strike. Furthermore the 

 structure of the constituents of the granitoid gneiss seems to have 

 undergone an entire change. In ordinary gneiss many of the 

 particles, as to form and composition, seem in a measure to partake 

 of their original nature, while in the granitoid gneiss all seem to be 

 crystalline, while the felsitic portions are developed into felspar and 

 quartz. Perhaps hereafter Microscopists will point out that such 

 rocks should be classed as Foliated granite, and removed from the 

 Gneiss series. Nevertheless the granitoid-gneiss or foliated-granite 

 will always be the passage-rock between gneiss and typical granite. 



The granite due to extreme metamorphism of derivate or sedi- 

 mentary rocks, as pointed out by Haughton and others, is very 

 varied in composition on account of the diversity of the rocks from 

 which it is formed ; and nearly all the changes just enumerated have 

 been previously pointed out by MacCuUoch, except that he has not 

 mentioned the peculiarities of the foliation of the granitoid gneiss. 



The historian of the Scotch rocks has grouped among his Granites 

 certain intrusive rocks, on account of their aspect, make, and position. 

 This grouping to a certain extent is not unnatural, as the rocks have 

 a granitoid aspect and occur as intrusive masses ; nevertheless many, 

 if not all of them, in favourable localities, will be found to be 

 metamorphosed igneous rocks. 



In the portion of Galway before referred to, it has been found 

 that some of the high silicious felstones (Petrosilex), many of the 

 basic-fel stones (Eurytes of Daubuisson, included in the Hybrid rocTcs 

 of Durocher), also some of the Whinstones or basic igneous rock 

 (BoUrite, MelapJiyre, Diabase, etc.), more especially if affected by a 

 scaly, platy or cleaved structure, will become first, schistose; second, 

 gneissic; and lastly, granitic. The homogeneous or compact varieties 

 however, generally change by gradually becoming more and more 

 highly crystalline and granitoid in aspect, till eventually they are 



• ^ In the description of the Geology of Abyssinia (Geology and Zoology of Abys- 

 sinia, 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 eyidently 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 the 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. 



