STRATIFICATION. 



83 



sponding inclination — usually 5° to 8°, but sometimes steeper. When 

 the sands are coral or shell sands, they become cemented into a calca- 

 reous sand-rock. 



b. Ebb-and-floio structure — This kind of 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 Fig. 61 e. The succession indicates frequent changes in the currents 

 during the deposition, such as attend the ebb and flow of tidal or river 

 currents over a shallow bottom. The oblique lamination, observed in 

 three of the layers of this figure, arises from a strong flow pushing up 

 the sands before it. 



When the flow is accompanied by powerful waves or plunging ,of 

 the water, the thrust at each plunge, besides bearing off part of what 

 had before been laid down, deposits the sand in successive portions, as 

 in Fig. 61 f, each obliquely and divergently laminated; each such por- 

 tion is often two or three yards long and six inches or a foot thick, 

 but varying much. Beds having this floio-and-plunge structure may 

 alternate with others horizontal in bedding. In obliquely laminated 

 beds, the lamination dips toward the direction from which the current 

 came. 



c. Sand-drift structure. — The layers consist of subordinate parts 

 dipping in various directions (Fig. 61 d), as if a laminated hillock made 

 by sand drifted by the winds on a coast (for the sands of such drifts are 

 always in layers) had been partly carried away, and then other layers 

 had been thrown over it by the drifting winds at a new inclination, and 



Fig. 62. Fig. 63. 



this violent removal and replacement had been often and variously 

 repeated. Fig. 61 d, representing this mode of structure, is from 

 Foster &, Whitney's " Report on the Sandstone Rocks of Lake Supe- 

 jrior." Fkf. 61 e is also from the same work. 



