352 ARGILLACEOUS SCHIST. 



fragments, more or less regular, and presenting 

 great diversity of form. 



They are moreover very frequently intersected 

 by numerous and minute veins of quartz or of 

 calcareous spar ; which, in the case of contortions, 

 frequently follow the flexures of the schist in 

 which they lie ; a fact of considerable interest in 

 a geological view. 



The essential minerals of the argillaceous 

 schists, are the peculiar indurated clay which 

 by itself forms all the simple varieties, together 

 with quartz and mica, which enter into the coarser 

 or compound kinds. The conglomerated va- 

 rieties, or the coarse graywackes, contain, in 

 addition to these, fragments of some of the pri- 

 mary rocks. It ought also to be added, that in 

 some rare instances, grains of felspar occur in 

 such a manner as to give the rock a porphyritie 

 appearance. 



According to these different circumstances, 

 the texture of the several varieties differ; the 

 finer, presenting one which is perfectly compact 

 and uniform, while the micaceous, the sandy, 



