SILICEOUS SCHIST. 560 



always present in both. Yet, in both instances, 

 as the change in the original strata may be pro- 

 duced by the passage of veins of granite or trap, 

 as well as by the contact of exterior masses, it is 

 easy to understand how this substance may occur 

 in a very limited manner, or in portions of strata. 



It moreover happens in the siliceous schists 

 of the_secondary class, that the stratified form 

 diminishes in regularity till it nearly or altogether 

 disappears ; in which case they have sometimes 

 been confounded with the basalts to which they 

 then become intimately allied ; giving rise to 

 errors, which it is the business of a geological 

 treatise to examine in a manner not here admis- 

 sible. The most important of these is the ima- 

 ginary existence of organic remains in basalt. 



The only peculiarity of internal structure de- 

 serving of notice which occurs in the siliceous 

 schists, within my observation, is the spheroidal 

 concretionary ; but, as far as I have observed, it 

 is more remarkable in the secondary than in the 

 primary. In the former, it is common in Sky 

 and in the Shiant Isles, the original and con- 

 nected strata being shale : in the latter, it is found 



