652 PRINCIPLES OF STRATIGRAPHY 



wholly composed of grains of calcium carbonate, with or without 

 magnesium. 



As Shaler has pointed out, the sand of the seashore is com- 

 pacted into a resistant mass by the films of water which separate 

 the grains and which are held in position by capillary attraction. 

 Whoever has walked on wet beach sand has noted the difference in 

 firmness between the wet and dry sands, the former often con- 

 stituting a hard, level floor. On such a surface the force of the 

 waves is spent ; the sands will retain their original angular charac- 

 ter, since the dividing film of water acts as a cushion, which pre- 

 vents the mutual attrition of the grains. Thus the grains of beach 

 sand are normally angular and with fresh surfaces, and this type of 

 sand should be looked for in normal marine sandstones. (Shaler- 

 88.) Shaler cites as an example of marine sands protected in this 

 manner from wearing, the sand of northern Florida . . . "which 

 has traveled southward from the region beyond Cape Hatteras" 

 . . . and which "is not more rounded than much which is in the 

 inner or landward dunes of the coast within sound of the ocean 

 waves." (88:/5/.) 



When, through drying, the binding films of water are destroyed, 

 the sands become loosened and are then readily shifted about by 

 the wind, accompanied by mutual attrition of the grains. Here, 

 then, no permanent structural features are formed. Both rill and 

 ripple marks left on the retreat of the tide are either obliterated by 

 the wind or washed away by the returning tide, owing to the non- 

 coherency of the material. An exception to this seems to be the 

 wave mark on a very gently sloping sand beach, and the hollows ex- 

 cavated behind pebbles or shells by the return of the wave on such 

 a beach. Examples of these are known from the Upper Medina 

 sandstone in western New York, and in other formations. (Fair- 

 child-32.) 



Marine arkoses. Accumulation of feldspathic sands on the sea 

 coast and their incorporation in marine strata are effected under a 

 peculiar combination of circumstances such as exist to-day in the 

 Gulf of California, as described by AIcGee (57). The granitoid 

 rocks of this region are subject to disintegration under the arid 

 climatic conditions, due to the interception of the Westerlies by the 

 coast ranges. Decomposition is practically absent, the disintegrated 

 material being transported by sheet floods. These result from ex- 

 ceptional thunderstorms, accompanied by sudden and extensive pre- 

 cipitation. Part of the material is carried into the Gulf and there 

 assorted by the waves, the coarsest and cleanest material being de- 

 posited at the salients of the coast, while in the reentrants much 



