KiXAHAX — On Granitic and other Ingenite Rocks. 133 



Pakt II. 



Suggestions to account for the Origin of the Granitic and other Sypogene 



Rochs. 



In the preceding part of this essay it was demonstrated that the 

 "whinstones may gTadnate through eiu'yte into felsyte {^'petro-silex or 

 highly siliceous felstone), "while these plutonic rocks (whinstones and 

 felstones) graduate into the granitic rock and tlii'ongh elvanyte into 

 typical granite ; the gTanitic rocks being of different types, some being 

 more basic than others ; and as there are basic and highly siliceous plu- 

 tonic rocks, so also in the granitic rocks some elvanytes and granites are 

 basic, while others are highly siliceous. It has also been shown that 

 all nonnal rocks, whether derivate or igneous, may be metamoi'phosed ; 

 at first becoming schistose, second gneissic, and finally granitic, the 

 granites yarying in accordance with the natui'e of the constituents 

 composing the rocks preyious to their being metamoi-phosed. 



Before proceeding fiu'ther, and for the convenience of reference 

 hereafter, the ingenite rocks in this area may be divided into seven 

 gToups, namely, i., the schist-series; n., the gneiss-series ; m., the non- 

 intrusive-oligoclasic granite; iv., the intrusive-oligoclasic granite; v., the 

 orthoclasic granite ; vi., the elvanyte ; vn., the plutonic rochs'^. 



It seems to be now generally allowed, that all granitic and other 

 ingenite rocks were formed fi'om previously-existing rocks, the hypogene 

 at a depth beneath the earth's siuface, but the volcanic and plutonic 

 rocks at or close to the present or a former surface of the globe. 



Some of the hypogene rocks {granitic rochs) must at one time have 

 been more or less fused and liquified, while others {gneiss and schist) 

 were never fused or liquefied, but were more or less changed by heat, 

 either wet or dry. Wet heat, judging fi-om the rocks of West Galway 

 and Mayo, would seem to be the more probable ; for in all the schist, 

 gneiss, and non-intrusive granite, the original joint-lines, as previously 

 mentioned, are either sealed, or a thin portion of the rock adjoining the 

 joint walls is silicefied and indurated, as if by being percolated by steam 

 charged with silex or some such substance (figs. G-. H. PI. 9 and P. 

 PI. 10). 



If we suppose that at a certain distance beneath the sm-face of the 

 earth all rock becomes fused and liquefied, the rocks in the zone above 

 the meltiag point would be fully mineralized, but not liquefied, wliile 



* In this table the metamorphosed plutonic rocks might be ignored, as they 

 do not affect the following arguments — nevertheless, all of them, excepting the 

 metamorphic orthoclasic granite, are included in the above classification. The 

 hypogene rocks, as the name implies, were fonued at a depth beneath the surface of 

 the earth, and under a pressui-e more or less great ; the granitic rocks having been 

 fused and subsequently cooled, and consoKdated, prior to their being exposed by 

 denudation at the present surface of the earth. 



K. I. A. PROC, SER. II., VOL. II., SCIENCE. T 



