SILICEOUS SCHIST. 279 



dental or local variation of those schists with which it 

 is associated, and owing that change to the influence 

 of other neighbouring masses of rock. It is equally 

 unnecessary, here, as in, the case of jasper, to form 

 two divisions ; although the characters of this sub- 

 stance differ in the two classes. 



When siliceous schist occurs among the primary 

 strata, it forms a portion of the beds of argillaceous 

 schist with which it is connected ; and as these vary 

 in their general characters, it presents corresponding 

 differences. It does not appear ever to form extensive 

 masses, as has been asserted, or to be an independent 

 rock, like argillaceous schist. This error has arisen- 

 from deducing the nature of such a mass from the 

 examination of a casual specimen ; or from investi- 

 gating those few beds only of a body of schist, which 

 happened, from their situation, to put on this cha- 

 racter. If the position of that portion of any mass of 

 primary argillaceous schist which exhibits the cha- 

 racter of siliceous schist, be examined, it will be found 

 to lie in the immediate vicinity of granite or porphyry, 

 'whether in masses or veins, or of the more recent trap 

 rocks. If, in any case, such causes are not apparent, 

 we know that these intruding rocks often lie imme- 

 diately beneath the surface, though invisible, and have 

 often been removed, by posterior waste, from the places 

 which they once occupied. Numberless facts occur- 

 ring in granitic districts prove that this is the true 

 theory of siliceous schist ; while it is confirmed by the 

 analogy of those found among the secondary strata, 

 produced only when masses of trap come into contact 

 with the soft shales. And, that the cause consists in 

 the action of heat, is justified by chemical experiment, 

 as well as by the peculiarity of their position with 

 respect to the igneous rocks, and by the gradation 



