123 



Whether the foliated rock admits of being di- 

 vided or not, it often exhibits marks of curvature, 

 on a great and on a small scale; sometimes 

 amounting to contortions. 



The schistose structure is distinguished by the 

 facility with which the rock splits into laminae by 

 mechanical force. A schistose rock rarely, if 

 ever, exfoliates by the mere action of the weather. 



In some cases, the schistose structure is found 

 in a simple rock, as in fine argillaceous schist, 

 (clay slate). In these, it appears at times to be 

 the result of mechanical deposition ; being paral- 

 lel to the stratification : at others, being oblique 

 to that, it seems to arise from a concretionary ar- 

 rangement of the particles, analogous to that by 

 which the spheroidal forms are produced. 



In other cases, the schistose structure exists in 

 aggregate rocks, even of a very coarse texture, or 

 containing large nodules ; and, in those, it appears 

 to be dependent on the simple rock which forms 

 the basis of such mixed masses. Coarse gray- 

 wackes and conglomerates, present examples of 

 this occurrence. 



