558 SILICEOUS SCHIST. 



be found, that, in these situations, it is frequently 

 in the immediate vicinity of granite or of por- 

 phyry ; or that, at least, the actual existence of 

 the first of these rocks, or the former existence of 

 the last, may be inferred where they are not 

 at present visible. 



In the secondary class, siliceous schist occu- 

 pies situations similar to that which jasper does, 

 and on which it is unnecessary to dwell ; particu- 

 larly, as it would lead to geological disquisitions 

 far too important to be lightly passed over, and in 

 a great measure foreign to the present object. It 

 is here sufficient to say, that, in these cases, it is 

 covered or intersected by trap rocks, and con- 

 nected with shale, into which it passes by imper- 

 ceptible transitions. Hence it also presents 

 varieties dependant on the varying character of 

 the original strata, which will be noticed in the 

 Synopsis ; and, for the same reason, it is found 

 associated both with the cherts and jaspers that 

 owe their origin to the same cause. 



From the circumstances under which the 

 siliceous schists occur, both in the primary and 

 secondary classes, the stratified form is nearly 



