452 
ORIGIN OF SAND. 
were derived were wholly, or even chiefly, quartzose in char¬ 
acter ; for, in many composite rocks, as, for example, in the 
granitic group, the mica, felspar, and hornblende are more 
easily decomposed by chemical action, or disintegrated, com¬ 
minuted, and reduced to an impalpable state by mechanical 
force, than the quartz. In the destruction of such rocks, there¬ 
fore, the quartz would survive the other ingredients, and 
remain unmixed, when they had been decomposed and had 
entered into new chemical combinations, or been ground to 
slime and washed away by water currents. 
The greater or less specific gravity of the different constit¬ 
uents of rock doubtless aids in separating them into distinct 
masses when once disintegrated, though there are veined and 
stratified beds of sand where the difference between the upper 
and lower layers, in this respect, is too slight to be supposed 
capable of effecting a complete separation.* In cases where 
rock has been reduced to sandy fragments by heat, or by 
obscure chemical and other molecular forces, the sandbeds 
may remain undisturbed, and represent, in the series of geo¬ 
logical strata, the solid formations from which they were 
derived. The large masses of sand not found in place have 
been transported and accumulated by w r ater or by wind, the 
former being generally considered the most important of these 
agencies; for the extensive deposits of the Sahara, of the des¬ 
erts of Persia, and of that of Gobi, are commonly supposed to 
have been swept together or distributed by marine currents, 
and to have been elevated above the ocean by the same means 
as other upheaved strata. 
* In the curiously variegated sandstone of Arabia PetraBa—which is 
certainly a reaggregation of loose sand derived from particles of older 
rocks—the contiguous veins frequently differ very widely in color, but not 
sensibly in specific gravity or in texture; and the singular way in which 
they are now alternated, now confusedly intermixed, must be explained 
otherwise than by the weight of the respective grains which compose 
them. They seem, in fact, to have been let fail by water in violent ebul¬ 
lition or tumultuous mechanical agitation, or by a succession of sudden 
aquatic or aerial currents flowing in different directions and charged with 
differently colored matter. 
