472 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



water of higher than ordinary temperature. The quartz, the carbon- 

 ate of lime, and the argillaceous matter above mentioned are pecu- 

 larly at home in the Primitive Slate formation, and are compara- 

 tively rare in the fundamental gneiss or primitive formation- 

 proper. We have only to refer to the highly quartzose rocks of 

 the Huronian formation, of the Thelemarken quartz formation,, 

 and of the so-called primary sandstones of the western islands of 

 Scotland, to show that the separation of quartz on an extraordina- 

 ry scale must have been one of the first products of the condensa- 

 tion of aqueous vapour on the eartti's surface. Moreover, although 

 primary limestones are not of unfrequent occurrence in gneiss, 

 they are of trifling extent compared with the limestones of the so 

 called Primitive Slates. At first of less frequent occurrence, of light 

 grey colour, and crystalline character, and evidently more the re«- 

 suit of a chemical precipitation than made up of animal organisms, 

 they pass through various gradations of color, becoming more 

 frequent and of darker color (more charged with carbon) as they 

 grow younger. In the micaceous and the clay slates, which ex- 

 ceed in extent of developement both quartzites and limestones, we 

 find a similar gradual change in their colours and lithological 

 characters ; the younger they become the more they are charged 

 with carbon, and the more they resemble slates of more modern 

 formations. The source of this carbon was undoubtedly the at- 

 mosphere, where it probably existed free, or was derived from the 

 decomposition of its compounds with other elements. Duringthe 

 period, when the primitive slate rocks were formed, the metallic 

 chlorides were also most probably removed from the atmosphere. 

 This may have given rise to the extensive metallic deposits exist- 

 ing among these crystalline slates. 



After the abrasion of the material from which the quartzose? 

 micaceous and argillaceous slates resulted, we must suppose that it 

 became deposited in the hollows of the then existing crust, which 

 hollows were most probably occupied by primitive strata lying 

 horizontal or nearly so. Those parts of the first crust, which rose 

 above this primitive ocean, are most likely to have been the high- 

 ly inclined primitive strata or eruptive masses of granite. If 

 this view be correct then the rocks of our Transition formation, 

 must generally have been deposited conformably upon horizontal 

 gneiss, or rocks allied to it. 



While the atmospheric agencies, and more especially water, 

 were thus at work upon the surface of the original crust of the 



