85 



places in a geological arrangement, and of which 

 two, at least, have been hitherto enumerated only 

 among minerals: These are, siliceous schist, jas- 

 per, and chert. As far as can be ascertained, 

 from the circumstances attending their geological 

 positions, they are all of a local nature, and ap- 

 pear to be modifications occurring at the com- 

 mon boundary of the stratified and the unstratified 

 rocks ; being apparently produced, in most cases, 

 by the local influence of the latter. As they be- 

 long equally to both the primary and secondary 

 classes, they have been placed, as already re- 

 marked, under a distinct title. This expedient ap- 

 peared preferable to that of enumerating them 

 merely among the varieties ; where however they 

 are also noticed when necessary. Siliceous schist 

 indeed has long been considered, like argillaceous 

 schist, a peculiar and distinct formation, and its ab- 

 sence might therefore have excited surprise. 

 Moreover, on the view of its geological history 

 here adopted, it became absolutely necessary, in 

 giving it a place, to allot one also to each of those 

 substances which bear so striking an analogy to 

 it in their origin and connexions. 



