STRATIFIED ROCKS, 



19? 



FIG. 114. Flattened 

 nodules J., face- 

 view ; .5, side-view. 



tion of the thickness of the slate or at right angles to 

 cleavage. On the cleavage surface the shape is broad, 

 elliptical (Fig. 114, A], while on sec- 

 tion the shape is very flat, B. These B 

 spots before mashing were round pellets 

 of clay. They have been mashed into an 

 ellipsoid of three unequal diameters, the 

 longest, a 5, in the dip of the cleavage, 

 and therefore nearly vertical ; the next, 

 c d, in the strike of the cleavage, and 

 therefore horizontal ; and the smallest, 

 e /, at right angles to cleavage. This 

 proves that the whole mass has been 

 mashed at right angles to cleavage, and 

 extended in the direction of the dip of cleavage. Micro- 

 scopic examination shows that every constituent granule 

 of the original clay is in the slate mashed into a thin scale, 

 so that the original granular structure is changed into a 

 scaly structure, and it is this which determines the easy 

 splitting. 



Geological Application. The amount of mashing to- 

 gether horizontally and extension vertically shown in these 

 different ways is so great that an original cube or sphere 

 in the unsqueezed mass is changed into an oblong, of 

 which the shortest diameter is to the longest as one to 

 three or four, one to five or six, one to nine or ten, and 

 even sometimes as one to fifteen. The average in well- 

 cleaved slates is one to six. Now, when we remember that 

 thousands of square miles and thousands of feet thickness 

 of rocks are thus affected, it is evident that this slow 

 mashing together horizontally of whole mountain-regions 

 must be an important agent in the elevation of land, and 

 especially in the formation of mountains. We shall speak 

 of this again under the head of mountains. 



