546 ORIGIN OF SAND. 
mixed, when they had been decomposed and recomposed into 
new mineralogical or chemical combinations, or been ground 
to slime and washed away by water currents. 
The greater or less specific gravity of the different constitu- 
ents 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 ob- 
scure chemical and other molecular forces, the sand-beds may 
remain undisturbed, and represent, in the series of geological 
strata, the solid formations from which they were derived. 
The large masses of sand not found in place have been trans- 
ported and accumulated by water or by wind, the former being 
generally considered the most important of these agencies; for 
‘the extensive deposits of the Sahara, of the Arabian peninsulas, 
-of the Llano Estacado and other North and South American 
deserts, of the deserts of Persia, and of that of Gobi, are sup- 
posed to have been swept together or distributed by marine cur- 
rents, and to have been elevated above the ocean by the same 
means as other upheaved strata. 
Meteoric and mechanical influences are still active in the re- 
duction of rocks to a fragmentary state; + but the quantity of 
*Tn the curiously variegated sandstone of Arabia Petraea—which is certain- 
ly a reagegregation of loose sand derived from disaggregation 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 al- 
ternated, 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 fall by water in violent ebullition or tumultuous mechanical 
agitation, or deposited by a succession of sudden aquatic or aérial currents 
flowing in different directions and charged with differently colored matter. 
+ A good account of the agencies now eperative in the reduction of rock to 
sand will be found in WINKLER, Zand en Duinen, Dockarm, 1865, pp. 4-20. 
I take this occasion to acknowledge my obligations to this author for assuming 
the responsibility of many of the errors I may have committed in this chapter, 
by translating a large part of it from a former edition of the present work 
and publishing it as his.own, 
