128 GEOLOGY. 



Fig. 49. 



ly appear in mountains, forming often their central part, 

 while the stratified abound in plains, or flank the sides 

 of the mountains. When a mass of unstratified rock, as 

 granite, stands up as the axis or central part of a mount- 

 ain, stratified rocks commonly slope on from the granite, 

 this having thrust them to the one side and the other as 

 it rose up out of the bowels of the earth. 



There is one class of unstratified rocks, the trappean, 

 noticed in 152, which have a tendency to regularity of 

 shape, and in some cases the tendency is fully carried 

 out. In this they are distinguished from the shapeless 

 masses of granite and other unstratified rocks. 



212. Stratification, Lamination, Joints, and Cleavage. 

 The word layer is often used as meaning the same thing 

 as stratum, but in the strict use of the latter term it in- 

 cludes all the layers of the same kind which are next to 

 each other. A stratification means a succession of lay- 

 ers, either of the same kind or of different kinds. Lay- 

 ers differ very much in thickness, according to the man- 

 ner in which they were laid down. When they are very 

 thin they are called laminae. In shales and micaceous 

 sandstones the lamina? are so thin that we may properly 

 speak of them as films, and it is plain that in such cases 

 the stratum or bed was formed by the very gradual ac- 

 cumulation of films of clay or of micaceous spangles, 

 which settled down at the bottom of comparatively still 



