AND ANALOGIES, OF ROCKS. 211 



those portions which come into contact with the 

 latter, become, first, siliceous schists, and, ultimately, 

 hornblende schist; so that the very same bed which 

 is an interlamination of gneiss and clay slate in one 

 part, is, in another, the usual alternation of gneiss and 

 hornblende schist. 



We thus lastly arrive at gneiss; a rock which often 

 bears the marks of igneous consolidation in a still 

 greater degree than those of aqueous deposition, but 

 in which it is almost unquestionable that both have 

 been combined. Where gneiss is at a distance from 

 granite, its laminar and stratified disposition is most 

 perfect; where in its vicinity, that is most obscure; 

 so obscure indeed as at length to disappear. This 

 is precisely what might be expected to happen on 

 this view of its double origin ; namely, the application 

 of heat in unequal degrees, to a series of beds deposited 

 from water, and probably, like quartz rock, originally 

 consolidated from it also. Where it is most remote 

 from granite, although its mineral materials should 

 be the same, they are disposed in a different manner; 

 or are more rigidly laminar and more independent. 

 Where it is most immediately in the vicinity of that 

 rock, and more particularly when it abounds in gra- 

 nite veins, the structure becomes analogous to that of 

 granite, or to one in which there is that mutual pene- 

 tration of crystals which can take place only in a fluid 

 of fusion. At length it actually passes into the con- 

 tiguous granite; losing that parallelism of the parts, 

 and those last remains of the laminar disposition, which 

 had gradually been decreasing. 



It is by no means difficult to imagine this combi- 

 nation of causes and of effects: a state of softening 

 or semifusion, sufficient to allow the integrant parts 

 of a stratified watery deposit to enter into new com- 



P 2 



