CLIMATE AND TERRESTRIAL DEPOSITS 273 



of the rock. The contrast between the river deposits of humid and 

 subarid regions is well brought out by the comparison of the alluvium 

 of the Indo-Gangetic plain with that of the Brahmapootra in Assam, 

 the material in both cases coming from the same mountain system.' 



The most marked chemical distinction of subarid flood-plain 

 deposits from those of truly arid regions is found in the small quantity 

 of evaporation deposits of calcium carbonate, gypsum, and salt, but 

 especially of the two latter. Lime may be quite abundant, as shown 

 by the Kankar of the Indo-Gangetic plain, its importance depending 

 largely upon the quantity in solution in the river water. Gypsum 

 and salt, however, formed by the evaporation of salt lagoons bordering 

 the sea, are largely washed out by the rains and floods of the rainy 

 season. As gypsum and salt impregnations of clay strata they may 

 be preserved and sometimes as purer deposits of salt, as illustrated by 

 the deposits now occasionally formed on the Rhone delta, as noted 

 long since by Lyell,^ in a climate which approaches subaridity. Such 

 deposits cannot be formed, however, in anything like the areal extent 

 or thickness with which they may occur in truly arid regions. Gypsum 

 in delta deposits is less an indication of aridity than salt, since the 

 former is precipitated upon the evaporation of 37 per cent, of normal 

 sea water while the precipitation of salt only begins when 93 per cent, 

 has been evaporated. It is to be noted, however, that the majority 

 of recent sediments containing gypsum are found in arid climates, 

 and where occurring as impregnations in ancient deposits which 

 were not laid down in contact with sea water would seem surely to 

 indicate a high degree of subaridity bordering upon truly arid condi- 

 tions. 



The alluvial soils of semiarid flood plains are particularly liable 

 to become deeply mud-cracked during the seasons of drought, but this 

 cracking may or may not be preserved in the sedimentary record. ^ 

 Over the regions of alternating sands and clays where the clay is not 

 calcareous the conditions are most favorable for the formation and 

 preserval of mud cracks. The importance of mud-cracking in further 

 drying out the soil and tearing the roots of plants has recently been 



1 E. W. Hilgard, Soils, 1906, pp. 410-14. 



2 Charles Lyell, Principles 0} Geology, 9th ed., 1853, p. 259. 



3 J. Barrell, Journal 0} Geology, Vol. XIV, pp. 528-33. 



