EVALUATION OF STRATIGRAPIIK' CRITERIA 435 



Cornish states that in wind -made ripples tlie coarser grains rest at the 

 crest of the ripples — in water they remain in the troughs. Tliese dis- 

 tinctions should be of aid in separating eolian and therefore terrestrial 

 from water-laid sands. 



RELATIONE or TEXTURI': TO SEDIMENTATION 



Degree of soiiing a negative crilerion. — On the whole, waves are nioi'e 

 effective agents for sorting than river currents, but since rivers wear 

 down gravel to sand in moving the bottom material downstream, all 

 degrees of wear and sorting may be found in their deposits also, and 

 distinctions founded alone on the degree of sorting are likely to lead to 

 false conclusions. It is rather criteria drawn from the shape and asso- 

 ciation of the particles which must be invoked to separate fluviatile and 

 marine material. But such distinctions if they are definite must await 

 field study, and in this place the discussion may be restricted to tlie 

 special case of the influence of wind in shaping material which enters 

 finally into fluviatile and marine deposits. 



Effects of ivind in shaping sand. — This topic has recently received 

 such full treatment by Sherzer,^^ who also gives abundant illustrations 

 and references to the literature on the subject, that discussion may begin 

 (m the basis of his paper. Sand grains subjected to either wind, water, or 

 glacial wear continued for a sufficient time come to consist almost wholly 

 of quartz, and each type of abrasion gives characteristic forms. Wind 

 wears fine sand much more rapidly than does water, moderate subjection 

 to wind action giving a high degree of sphericity, which extends to grains 

 which are less than a tenth of a millimeter in diameter, a size but little 

 affected by water action. Subaerial exposure of loose sands is thus soon 

 recorded in the form; but, as Sherzer notes, Sorby in 1880 called atten- 

 tion to the necessity of distinguishing between the age of the grains 

 themselves and the age of the deposit in which they may be found.-"^- 

 The same caution regarding age applies to the mode of origin of the 

 deposit as distinct from the mode of origin of the grains. The sands 

 which enter into both river and marine deposits may at some time in 

 their history have been subjected to the wind, and this may happen in 

 rather close relationship to the final- deposition. The relative oppor- 

 tunities for w^nd action in these two classes of deposits must therefore 

 be discussed and the problem raised of separating true desert deposits 



''I Criteria for the recognition of various types of sand grains. Hull. (Jeol. Soo. 

 America, vol. 21. 1010. pp. r)2r)-0()L'. 



■^ On the structure and origin of tli(> non-calcareous stratitied rocks. Troc. Quar. 

 Jour. Geol. Soc. London, vol. .\xxvi, 1880, p. .IS. 



