FACIES OF THE SHORE ZONE 653 



finely comminuted material is deposited with the coarse quartz and 

 feldspar. Quartz- feldspar-mica sandstones are thus produced 

 under conditions permitting the entombment of marine organisms. 

 Sorting of sands and gravels by zvaves. The conditions under 

 which waves accomplish sorting of sands and gravels according to 

 size and material are given by Bailey Willis (105:^6'/) as the fol- 

 lowing : 



(a) Vigorous wave action, accompanied by strong undertow. 



(b) Prolonged transportation in consequence of deep water 



and continuous currents. 



(c) Moderate volume of sediments. 



On the other hand, the conditions under which sorting is not or 

 but slightly accomplished are, according to Willis : 



(a') Feeble or diffused wave action. 



(b') Concentrated deposition. 



(c') Excessive volume of sediment. 



In general, with a given amount of loose materials to work 

 upon, the waves will accomplish sorting in proportion to their 

 strength and the strength of the undertow. The finest material will 

 be carried out farthest, while only the coarsest material will be left 

 behind. Where the material is all of one kind, as, for example, 

 quartz sand, the sorting will be entirely according to size, while 

 variation in the mineralogical character of the material may lead to 

 a sorting, according to the specific gravity as well. Thus quartz 

 sands may be entirely washed free from mica and clay particles, 

 while garnet and magnetite, two characteristic accompaniments of 

 sands derived from many igneous or metamorphic rocks, will segre- 

 gate through the washing out of all quartz grains. 



This sorting according to size and specific gravity is best accom- 

 plished if, in addition to the strong wave movement which stirs 

 up the sediment, strong currents exist which can transport the ma- 

 terial for long distances. The smallest and lightest material may 

 thus be carried much farther away from the point where the 

 coarsest is dropped, the separation being thus most pronounced. 

 In this respect the separation by currents will be analogous to the 

 separation of sediments by wind. A water current of a given 

 velocity is equaled in carrying power only by a wind current of 28 

 times that velocity (Udden-99: j/p), but wind currents exceeding 

 average ocean currents in velocity by very much more than that are 

 characteristic of many regions, especially in the upper atmosphere. 

 Thus material projected or carried into the air stands a much better 



