ATMOCLASTICS AND ANEMOCLASTICS 293 



arid climatic conditions mechanical disintegration will predominate, 

 especially in the case of crystalline rocks. Through differential ex- 

 pansion and contraction under heat and cold the minerals of the 

 rock will become separated and a mixed sand results, in which the 

 minerals have sharp outlines, owing to splintering along cleavage 

 planes. Reconsolidation of such a sand produces arkoses, which, 

 as in the case of the Torridon sandstone of Scotland, may have 

 all the appearance of an igneous rock. In moist climates chemical 

 alteration or decomposition of the feldspars (Mackie-iQ:^//^) and 

 other decomposable minerals will set in, resulting eventually in the. 

 production of clay, etc., in which quartz and mica abound. The 

 chief processes in this chemical destruction or decomposition of 

 rock minerals consist of oxidation (and deoxidation), hydration 

 (and dehydration), and carbonation; silication and desilication may 

 also occur and solution of minerals also belongs here. (Van Hise- 

 28:461.) Under rainy or pluvial conditions the clay and mica will 

 be removed, the latter suffering mechanical destruction, while the 

 quartz will become mechanically concentrated. When not modified 

 by subsequent water or wind activities, the quartz and other resist- 

 ant mineral grains will be found fresh and angular, without evi- 

 dence of subsequent rounding, while complete crystals of idio- 

 morphic minerals of the crystalline rock are among the resulting 

 constituents. When modified by subsequent agents, various sub- 

 types are produced, namely, hydro-atmoclastic, anemo-atmoclas- 

 tic, etc. 



4. The Anemoclastics. These are the wind-laid deposits 

 which are often of great extent, and are of great importance 

 to the stratigrapher. Ancmorudytcs are probably unknown, 

 but anemoarenytes and anemolutytes are widely distributed. 

 Some familiar examples are anemosilicarenytes represented 

 by solidified sand dunes of quartz sand ; anemocalcarenytes, 

 such as the consolidated wind-blown or seolian rocks of Ber- 

 muda in which the sand grains are wholly calcium carbon- 

 ate; and the complex anemolutytes forming extensive de- 

 posits of seolian dust, chiefly of volcanic matter, in the Ter- 

 tiary strata of North and South America, and in which some 

 of the best preserved remains of mammals have been found. 

 These dej)osits approach and grade into the pyrolutytes which 

 form in volcanic regions, and in fact it becomes a matter of 

 opinion where the line between the two is to be drawn. The 

 wind-laid loess is also a good example of an impure anemo- 

 lutyte. 



