526 OEiaiN OF SAJS^D. 



binatious, or been ground to slime and washed away by water 

 currents. 



The greater or less specific gravity of the different eonstituenta 

 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 shght 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 sand-beds may remain undisturbed, 

 and represent, in the series of geological strata, the solid forma- 

 tions from which they were derived. The large masses of sand 

 not found in place have been transported 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 JS'orth and South American deserts, of the deserts of 

 Persia, and of that of Gobi, are 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.f 



Meteoric and mechanical influences are still active in the 

 reduction of rocks to a fragmentary state ; :}: but the quantity of 

 sand now transported to the sea seems to be comparatively incon- 

 siderable, because — ^not to speak of the absence of diluvial ac- 



* In the curiously variegated sandstone of Arabia Petrsea — wMch is certainly 

 a reaggregation 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 

 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 fall by water in violent ebullition or tumultuous mechanical 

 agitation, or deposited by a succession of sudden aquatic or aerial ciurenta 

 flowing in different directions and charged with differently colored matter. 



f Klein, PhystscTie Geographie, p. 371. 



X A good account of the agencies now operative in the reduction of rock to 

 sand will be found in Winkler, Zand en Duinen, Dockum, 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. 



