134 Proceedhtgs of tJie Royal IrisJi Academy. 



each successive stratum would in general be less and less affected ;'^ 

 and if the rocks while in these conditions began to be raised, 

 or in any other way removed out of the influence of the heat, appa- 

 rently there would be conditions to account for the formation of all 

 kinds of ingcnite rocks. f 



The stratum that included all the rocks wliich were altered, but 

 not fused, would contain the metamorphosed rock ; the upper portion 

 of the rocks belonging to the schist-series, and the lower part to the 

 gneiss-series. Below this stratum, in the zone where the rocks were 

 fully mineralized, but not liquefied, there wouldbe a non-intrusive granite 

 rock formed, which would remain in its original position as regards the 

 overlying rocks, but its nature and constituents would be quite 

 changed. Below the melting point would be a mass of fluid, fi'om 

 which all the intnisive rocks would come. Turthermore, rocks formed 

 under such conditions would, as in natui'e, have no hard lines of de- 

 marcation, the lowest members of each graduating into the upper por- 

 tion of the group next below it. 



The formation of the non-intrusive groups of hypogene rock has 

 been suggested, but there still remain the intrusive rocks to be 

 accounted for. As the heat became less, the liquified portion would 

 cool and consolidate under various conditions, consequently forming 

 rocks not only different in aspect, but also in composition. At the 

 first, two magmas woidd form — a basic above, and a highly siliceous 

 below. As the superincumbent rocks cooled, cracks and other fissures 

 should form, into which portions of these magmas ought to be forced 

 by the pressure of the overlying mass, and also by the escape of pent- 

 up gases and steam. These intrusions, according to the magma from 

 which they came, ought to be basic or highly siliceous, and, if forced to 

 the earth's surface, they would cool and consolidate, under little or no 

 pressure, forming the different. varieties of phitonic and volcanic roclcs, 

 while the portions that solidified between their source and the earth's 

 sui-face would become elvanyte, a granite and hypogene rock, but not a 

 typical granite, as some of the silex crystallizes out before the other 

 rock constitiients. 



There now only remains the intrusive granites to be formed from 

 the residue of the fluid magmas, and the conditions under which these 

 would cool have to be considered. The more acid magma should cool 

 slower than the basic ; therefore, there ought to be two distinct rocks 

 forming the magma ; that cooled first becoming a basic rock {oligoclasic 

 granite), while tbe other, that took longest to cool, would form the 

 orthoclasic, or highly-siliceous granite. 



* In some rare instances, as pre'sdously mentioned, beds or portions of beds 

 migbt be changed into granite while the associated rocks above, below, and 

 around were only changed into gneiss or even schist. 



t Since this essay was read, Professor le Conte has published a most instructive 

 paj>er on the " Features of the Earth's Surface" (American Journal of Science and 

 Art, third series, vol. iv.), which bears on this subject, and to which the reader is 

 referred. \_Note in press.'] 



