130 GEOLOGY. 



is this arrangement that enables the workman to get out 

 readily square, or, rather, squarish blocks of stone. The 

 regularity of these divisions varies much in different 

 cases. Sometimes it is so great that the upper surface 

 of a stratum has very much the appearance of a regular 

 pavement made with nicely-fitted slabs. Where, from 

 any cause, the rocks are occasionally thrown down, ex- 

 posing new surfaces of the strata with their joints, there 

 is a resemblance to the ruins of fortifications. In Fig. 

 51 we have a view of some of the cliffs of Cayuga Lake, 



Fig. 51. 



N. Y., which weathering and the undermining action of 

 the water are continually wearing away, so that fresh 

 surfaces show in very definite manner the planes and 

 lines of division. Cleavage, or the slaty structure, is an- 

 other kind of division in rocks. This is sometimes par- 

 allel with the layers, and sometimes runs across them. 

 Both the larger divisions by joints, and the smaller ones 

 by cleavage, each often extend over vast regions of 

 country at the same angle, showing that some causes 

 operating extensively produce them. 



213. Order of Succession of Rocks. There is a regu- 

 lar order in the stratified rocks which is never trans- 



