DEPOSITS BY SPRINGS 12 J 



depend upon the force and direction of the wind, and often 

 imitate the confused arrangement of sands piled up by waves and 

 currents under water. The sand-grains of the dunes are, however, 

 more rounded by the abrasion which they have undergone, and, 

 especially in deserts, they are apt to be smaller. When the sands 

 are mixed with pieces of shells and other calcareous material, per- 

 colating waters, by dissolving and redepositing the CaC0 3 , may 

 cement the sands into firm rock. This is the more conspicuous 

 when the whole material is calcareous, as in the shell sands of 

 Bermuda. This substance, ground up by the surf, is transported 

 inland by the wind and piled up into dunes. Rain-water cements 

 the loose grains together, and by the alternate accumulation by 



liiiiil';l»i'.iii'V.', 





lllllksii 



Fig. 48. — Ideal section through Mammoth Hot Springs, showing the water rising 

 through limestone. (Hayden.) 



wind and cementing by rain is formed the stratified seolian or 

 drift-sand rock. 



Chemical Deposits. — As our knowledge of microscopic plants 

 increases, many processes which were believed to be purely chemi- 

 cal, are found to be dependent upon the activity of minute plants. 

 At present, it is not possible to distinguish accurately, in all cases, 

 between the two kinds of processes. 



Chemical deposits on the land are made principally by springs. 

 Many springs precipitate carbonate of lime, on coming to the 

 surface. The quantity of CaC0 3 which a given volume of water 

 will dissolve, depends upon the amount of C0 2 contained in that 

 water, and the quantity of dissolved gas, again, is determined by 

 the pressure to which it is subjected. When the spring-waters 

 reach the surface, the pressure is relieved, much of the C0 2 im- 



