82 



LITHOLOGICAL GEOLOGY. 



their mouths thousands of square miles in area, so in ancient time beds 

 of sand and clay were accumulated by these very means, and after- 

 ward consolidated into rocks. Again, as shells and corals, by grow- 

 ing in the ocean where shallow, under the action of the waves, pro- 

 duce the accumulating and rising coral-reef some hundreds of miles 

 long in the present age, so in former ages shells and corals grew and 

 multiplied and made coral-reefs and shell-rocks, and these old reefs 

 are the limestone strata of the world. The agency of water and life 

 in these great results is particularly considered under Dynamical 

 Geology. 



2. Structure of Layers. 



The structure of layers is due either to the original deposition of 

 the material, or to subsequent changes. 



(1.) Kinds of structure and markings originating in the act or 

 mode of deposition. — The kinds of structure are illustrated in the an- 

 nexed figures, and are as follow : a, the massive ; b, the shall/ ; c, the 

 laminated ; d, e, and/", the compound or irregularly bedded. These terms, 



Fig. 61. 



excepting the last, have been already explained (p. 63). The massive 

 is especially characteristic of pure sandstones and conglomerates. But, 

 if sandstones are argillaceous, that is, contain some clay, they are lam- 

 inated, and therefore break readily into slabs, like ordinary flagging- 

 stones ; and the thinness of the flags increases with the amount of 

 clay. A clayey rock is usually shaly. 



Compound structure is of different kinds. 



a. Beach structure. — The upper part of a beach, above high tide 

 level, is made by the toss of the waves, and especially in storms, and 

 is generally irregularly bedded, as represented in the upper part of 

 Fig. 61 e. But the lower part, swept by the tide, has usually an even 

 seaward slope ; and the beach deposits over it have therefore a corre- 



