THE ROCKS OP THE EARTH'S SURFACE SHELL 35 



Winds are under favorable conditions capable of transporting 

 both dust and sand, but not the larger rock fragments. The dust 

 deposits are found accumulating outside the borders of des- 

 erts as the so-called loess (Fig. 216), though the sand is never 

 carried beyond the desert border, near which it collects in wide 

 belts of ridges described as dunes. When this sand has been 

 cemented into a coherent mass, it is known as eolian sandstone. 

 A section of the appendix (B) is devoted to an outline description 

 of some of the commoner rock types. 



The different deposits of ocean, lake, and river. Of the sub- 

 aqueous sediments, there are three distinct types resulting: 

 (1) from sedimentation in rivers, the fluviatile deposits ; (2) from 

 sedimentation in lakes, the lacustrine deposits ; and (3) from sed- 

 imentation in the ocean, marine deposits. Again, the widest 

 range of character is displayed by the deposits which are laid 

 down in the different parts of the course of a stream. Near the 

 source of a river, coarse river gravels may be found ; in the middle 

 course the finer silts ; and in the mouth or delta region, where the 

 deposits enter the sea or a lake, there is found an assortment of 

 silts and clays. Except within the delta region, where the area 

 of deposition begins to broaden, the deposits of rivers are stretched 

 out in long and relatively narrow zones, and are so distinguished 

 from the far more important lacustrine and marine deposits. 



Lakes and oceans have this in common that both are bodies 

 of standing as contrasted with flowing water; and both are sub- 

 ject to the periodical rhythmic motions and alongshore currents 

 due to the waves raised by the wind. About their margins, the 

 deposits of lake and ocean are thus in large part wrested by the 

 waves from the neighboring land. Their distribution is always 

 such that the coarsest materials are laid down nearest to the shore, 

 and the deposits become ever finer in the .direction of deeper 

 water. Relatively far from shore may be found the finest sands 

 and muds or calcareous deposits, while near the shore are sands, 

 and, finally, along the beach, beds of beach pebbles or shingle. 

 When cemented into coherent rocks, these deposits become shales 

 or limestones, sandstones, and conglomerates, respectively. 



As regards the limestones, their origin is involved in consid- 

 erable uncertainty. Some, like the shell limestone or coquina 

 of the Florida coast, are an aggregation of remains of mollusks 



