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, 
