124 



Lastly, the schistose structure sometimes re- 

 sults from the laminar alternation of two distinct 

 substances, one of which is more yielding than 

 the other. This is found in some limestones, 

 but it may also be considered as a variety of the 

 lamellar modification. 



The thickness of the laminae produced by the 

 schistose structure is various ; it seldom, how- 

 ever, exceeds one, and rarely descends to the 

 twentieth of an inch. 



The laminar continuity is equally various. 

 It may extend to twenty feet, and is often limited 

 to a few inches, or even less. 



In these cases, the schists sometimes termi- 

 nate by extenuation ; at others, in consequence 



of transverse joints. These joints are frequently 



t 



so placed, and so straight, as to divide the lami- 

 nae into prismatic masses of different forms. 

 They are much more frequently oblique than 

 vertical to the laminar plane, and are often mu- 

 tually parallel. 



The laminae, or schists, are sometimes even ; 

 at others uneven, rough, or tubercular. 



They are generally straight, occasionally 



