278 STUDIES FOR STUDENTS 



as the massive travertine and caliche deposits of Arizona and Mexico.^ 

 In other cases, however, the content of lime may be no higher than 

 in the deposits of subarid flood plains, the percentage of dissolved 

 lime in the river water to undissolved detritus apparently having a 

 strong influence in this respect. For example, the deltas of both the 

 Rhine and the Rhone, especially the latter, show a considerable pro- 

 portion of calcium carbonate due to the highly calcareous nature of 

 the formations subjected to erosion, supplemented in the case of the 

 Rhone by the approach toward a semiarid climate over its delta in the 

 Mediterranean Sea, This delta consists in large part of sand cemented 

 by lime.^ In the deposits of the Nile delta, on the other hand, calcium 

 carbonate is probably no more or possibly even less abundant than in 

 the case of the Rhone. Analyses by Regnault of fresh Nile mud gave 

 22 per cent, of carbonates, of old Nile mud gave 11 per cent. Other 

 analyses by Knop from other localities gave, however, only 4.1 to 4. 7 

 per cent, of carbonates in the dried inorganic residues of Nile mud.^ 

 These may be compared with o. i per cent, of CaC03 as the average 

 content in three alluvial soils of the Ohio Valley and i . 38 per cent, of 

 CaC03 in two alluvial soils of the Mississippi Valley.'^ In the delta 

 muds of arid climates the proportion of true clay may be low, and 

 alkalis exist, largely either in the form of comminuted feldspar or as 

 soluble alkaline salts in the surface soil. Brine pools and gypsum 

 deposits will be not uncommon in the lower areas, especially near the 

 margins of the deltas. Dense reedy jungles and fever-breeding salt 

 swamps frequently dry at some season of the year may be common, but 

 the presence of evaporation deposits with the decolorized shales is 

 a characteristic which separates them from those of semiarid climates. 

 Those parts of the potash and soda which are dissolved from the 

 silicate minerals and form alkali crusts or flat lands in arid regions 

 are kept near the surface, being carried upward by capUlary action in 

 the dry season and washed downward a short distance by the occa- 



1 W. P. Blake, "The Caliche of Southern Arizona," Abstract, Engineering and 

 Mining Journal, Vol. LXXII, 1901, pp. 601, 602. 



2 G. R. Credner, "Die Deltas, ihre Morphologie, geographische Verbreitung und 

 Entstehungsbedingungen," Peterinann's Mitth. Ergdnzungshejt, No. 56, Vol. XII, 

 1878, p. 16. 



3 G. R. Credner, op. cit., p. 15. 



4 G. P. Merrill, Rocks, Rock-Weathering, and Soils, 1906, p. 351. 



