348 ARGILLACEOUS SCHIST. 



ficulty as micaceous schist ; in which, as already 

 remarked, it is often impossible to trace the 

 divisions of the strata. These separations be- 

 tween the beds, are sometimes caused by in- 

 tervening rocks of some other character ; but, 

 more frequently, they result from a change of 

 texture in the approximate parts of the general 

 mass. 



The strata of argillaceous schist are subject to 

 flexures similar to those which occur in mica- 

 ceous schist and gneiss; but the rock itself is 

 rarely contorted in the very minute manner in 

 which those sometimes are. 



It presents also some varieties of internal 

 structure, one of which constitutes its chief value 

 for the purpose of architecture. This is the schis- 

 tose disposition, in consequence of which it is 

 capable of being split into slates, of considerable 

 tenuity in many cases ; while, in others, it rather 

 presents imperfect indications of a fissile ten- 

 dency, than the property of dividing into conti- 

 nuous plates. This quality occurs both in the 

 finer and coarser varieties ; but the former pos- 

 sess it in the most perfect manner, although many 



