540 OCCASIONAL ROCKS. 



this rock has, from its mineral characters, been 

 often referred to a primitive or transition class, 

 where it formed a portion of the secondary ; thus 

 producing a total misapprehension of the relations 

 of the rocks with which it was connected. It is 

 not the only instance where similar geological 

 errors have been committed by determining the 

 class of a rock from its mineral character ; and 

 from thus concluding respecting the place in 

 which it ought to be found: it is one of those 

 which points out most strongly the utility, or 

 rather the necessity, of a geological arrangement 

 of rocks. 



That confusion has, in the primary siliceous 

 schists, been much increased by misapprehend- 

 ing even the true mineral characters of the sub- 

 stance in question, and by confounding it with 

 those varieties of argillaceous schist, which, from 

 containing a large proportion of quartz in their 

 composition, present an unusual degree of hard- 

 ness. 



Respecting chert, it is here proper to observe, 

 as formerly noticed, that is is now, for the first 

 time, introduced into a catalogue of rocks. The 



