360 J. BARRELL PLUVIATILE ORIGIN OP OLD RED SANDSTONE 



flood waters are dried out and cracked, the polygons resulting from the 

 cracking curling up on the edges. From the stream channels sand is 

 blown to leeward and buries, perhaps permanently, the dried and curled 

 plates of mud. The wind plays a much less important part, however, 

 than in true deserts, not so much because of the more restricted period of 

 action as because of the greater efficiency of the water and the binding 

 action of the vegetation of the plains. Clean sands, however, still give 

 rise to dunes, and even in humid climates dune action is not wholly absent. 



The significance of limestone deposited in fresh water needs to be con- 

 sidered. It is found to occur under a number of dissimilar conditions. 



First, over semi-arid or arid floodplains evaporation of ground water 

 takes place throughout much of the year. The dissolved material which 

 is most abundantly precipitated because of this evaporation is calcium 

 carbonate. Except under truly arid climates, the salt, gypsum, and alka- 

 line carbonates and sulphates are washed out by the following flood 

 waters ; but the lime, once precipitated, is relatively insoluble and remains 

 as a cement in the silt and sand. If the waters flow from limestone 

 regions, they are correspondingly richer in lime, and this process may 

 occur in relatively humid climates, as in that over the deltas of the Ehine 

 and Ehone. For the precipitation of lime in floodplain deposits from 

 waters flowing from crystalline rocks, it would appear, however, that the 

 climate must be at least semi-arid in its dryness. In India this cement 

 in the alluvial soils forms impervious nodular layers known as kankar; 

 in Mexico it occurs often as a granular impure limestone, called caliche, 

 and in the' western United States such sands and gravels cemented by 

 lime, chiefly at the upper level of the ground water, are known as mortar 

 •beds. Probably the agency of plants is inconsequential in this class of 

 lime deposits. 



Second, there are found to be deposited in fresh-water lakes and streams 

 lime muds, known as marl, or concretions of nearly pure calcium car- 

 bonate- In springs, salt lakes, and in tropic seas deposits of similar com- 

 position are also found. In recent years, in fact, it has become recognized 

 that Chara mosses, green and blue-green algse, and bacteria are the agents 

 of calcareous precipitation to such an extent that these lowly vegetable 

 forms are now thought by many to be quantitatively the most important 

 geological agents in the making of limestones. For the deposition of 

 fresh-water limestones, chiefly of algous origin, the only condition appears 

 to be that of some degree of warmth and of richness in calcium bicar- 

 bonate. Under the humid climate of the eastern United States such 

 marls have been extensively deposited in the lakes of northern Indiana. 15 



15 Blatchley and Ashley : The lakes of northern Indiana and their associated marl 

 deposits. Indiana Dept. of Geology and Natural Resources, 25th Ann. Rept., 1901, pp. 

 31-321. 



