366 STRATIFIED ROCKS [Ch. XXVI. 



Crag and other formations *. This disposition of the layers 



No. 89. 



Lamination of clay-slate, Montague de Seguinat, near Gavarnie, in the Pyrenees. 



is illustrated in the accompanying diagram, in which I have 

 represented carefully the stratification of a coarse argillaceous 

 schist, which I examined in the Pyrenees, part of which ap- 

 proaches in character to a green and blue roofing slate, while 

 part is extremely quartzose, the whole mass passing downwards 

 into micaceous schist. The vertical section here exhibited is 

 about three feet in height, and the layers are sometimes so thin 

 that fifty may be counted in the thickness of an inch. Some 

 of them consist of pure quartz. 



The stratification now alluded to must not be confounded 

 with that fissile texture sometimes observed in the older rocks, 

 by virtue of which they divide in a direction different both 

 from the general planes of stratification and from the planes 

 of those transverse layers of which a single stratum may be 

 made up. 



Another striking point of analogy between the stratification 

 of the crystalline formations and that of the secondary and 

 tertiary periods is the alternation in each of beds varying 

 greatly in composition, colour, and thickness. We observe, for 

 instance, gneiss alternating with layers of black hornblende- 

 schist, or with granular quartz or limestone, and the inter- 

 change of these different strata may be repeated for an indefinite 

 number of times. In like manner, mica- schist alternates with 

 chlorite-schist, and with granular limestone in thin layers. 



As we observe in the secondary and tertiary formations 



* See above, p. 173, 



