STRATIFICATION. 



93 



act or mode of deposition. — The kinds of structure are illustrated 

 in the annexed figure, and are as follow: — a, the massive; b, the 

 shall/; c, tlje laminated; d and e, the compound or irregularly bedded. 



Fig. 61. 



These terms, excepting the last, have been already explained (§ 80). 

 The massive is especially characteristic of pure sandstones and con- 

 glomerates. But if sandstones are argillaceous, that is, contain 

 some clay, they are laminated, or 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 or an im- 

 perfect slate. 



The compound structure is of three kinds, — the beach structure, the 

 ebb-and-flow structure, and the sand-drift structure. 



In the beach structure, as exemplified in d, the subordinate layers 

 are very irregular in thickness and extent, often thinning out at 

 short intervals and varying from pebbles or stones to sand and 

 clay. This structure is observed in any sea-beach where a cut has 

 exposed its interior arrangement. 



In the ebb-and-flow structure (e), the bed, although it be but 

 a few feet thick, consists of layers of various kinds, some of which 

 are horizontally laminated, and others obliquely so with great 

 regularity, as in the figure. The succession of members indicates 

 frequent changes or reversals in the currents during the deposi- 

 tion. Such changes attend the ebb and flow of the tides or tidal 

 currents or waves over a shallow bottom. 



In the sand-drift structure {/), the layers consist of subordinate 

 parts of very various lamination, one dipping in one direction and 

 another in another, as if a laminated hillock made by sand drifted 

 by the winds on a coast (for such sand-drifts are always in layers) 

 had been partly carried away, and then other layers been thrown 



