ARGILLACEOUS SCHIST. 191 



clay-slate and graywacke occupy considerable spaces, 

 without the intervention of the other rocks; limestone 

 also forming an occasional part of this series. I may 

 refer to the accounts of Chlorite schist and Primary 

 sandstone, for some other associations of this rock ; as, 

 under gneiss and hornblende schist, I have described 

 its peculiar connexions with those substances. I need 

 only add, that beds of argillaceous schist sometimes 

 occur in company with quartz rock and micaceous 

 schist ; in a very independent state, and far removed 

 from any masses of a similar rock ; as may be seen at 

 Balahulish. 



I should not be at the trouble of adding any thing 

 on the theory of argillaceous schist, since it is but that 

 of most of the preceding strata, had it not been mis- 

 apprehended and disputed. That both the fine and 

 the coarse were originally deposited from water, must 

 be apparent, as must the nature of the original mate- 

 rials. But there are cases in which it must subse- 

 quently have been exposed to heat, if it has not in 

 every one; since, in this way, and no other, can be ex- 

 plained the formation of the minerals of igneous origin 

 within it, and indeed of any minerals belonging to its 

 texture. The case is that of micaceous schist and gneiss; 

 and thus too are the porphyritic graywackes explained. 

 In the same manner, probably, must be explained its 

 several concretionary structures. It is also connected 

 with the theory of this rock and of micaceous schist 

 both, to remark, that at its contact with granite, it is 

 often converted into the other for a small space, as it 

 is also, if more rarely, into gneiss; while if any one 

 still continues to maintain that these are distinctly stra- 

 tified substances, and not cases of igneous influence, 

 the theory is proved by the fact that the same changes 

 occur where granite veins penetrate, in a manner which 



