294 PRINCIPLES OF STRATIGRAPHY 



In form, anemoclastic sand grains are apt to be more thor- 

 oughly rounded and worn than similar grains worn by water. In 

 general, rounding of superfine sand, i. c, grains below o.i mm., is 

 not accomplished by water (Chamberlin and Salisbury-i li-^d ; 

 Daubree-5 ri'jd), but such grains may readily be rounded in air. 

 As shown by Mackie (see Chapter V), particles of quartz sand, 

 less than one-fifth the diameter of those rounded by water, will be 

 rounded to an equal extent by wind. Pitting and frosting of the 

 surface is another characteristic result of seolian activity, as is 

 also assortment according to size of grain aud specific gravity of 

 mineral, so that in a typical aeolian sand the grains are of approxi- 

 mately uniform size, and of the same mineral throughout, generally 

 quartz. 



Most aeolian sand and dust is derived from some other type of 

 sand, such as residual material (anemoatmoclastic), glacial sand 

 (anemoautoclastic), river or beach sand (anemohydroclastic), or 

 volcanic sand (anemopyroclastic). Extensive deposits of anemobio- 

 clastic sands may accumulate around quarries, etc., especially where 

 stone is crushed for road material. WHiere the reworking by wind 

 has been extensive, the evidence of the original character may be 

 destroyed, ^olian sand in a state of rest may have its grains 

 coated with iron oxide, as in the case of the red sand of the 

 Arabian desert. (Philipps-23 ://o.) 



5. The Hvdroclastics. These comprise by far the larger num- 

 ber of clastic rocks. They are the water-laid deposits and 

 include the following common types : 



a. Hydrurudytcs, or conglomerates of variable compo- 



sition. 



b. Hydrosilicinidytcs, or pure quartz conglomerates, and 



various varieties due to admixture of simple mineral 

 matter. 



c. Hydrocalcirudytcs. or pure lime conglomerates and the 



varieties due to iron, silica or other simple impurities 

 in the paste. 



d. Hydrarenytcs, or water-laid sandstones of variable com- 



position. 



e. Hydrosilicarciiytcs, or pure quartz sandstones with va- 



rieties due to simple admixtures in the paste. 



f. Hydrocalcarcnytes, or pure lime sandstones with vari- 



eties as above. 



g. Hydrolutytcs, or water-laid mud beds of variable com- 



position. 



