CONDITIONS OF SEDIMENTARY DEPOSITION. S°7 



ditions under which lime is carried from the land, and to consider 

 how it is distributed in the sea. As was stated early in this paper, 

 the amount of lime carried annually from a given land area is 

 directly related to the efficiency of rock-decay7 rock-decay is 

 most efficient over surfaces which have suffered prolonged de- 

 gradation, and on such surfaces the development of drainage 

 systems has usually resulted in the growth of great rivers. 

 Hence the lime contributed from continents to oceans is delivered 

 chiefly at a few places, the mouths of extended systems, and 

 there is great inequality in the distribution of these along differ- 

 ent coasts and among different seas. Of this fact South 

 America is the most conspicuous example, with all its great 

 rivers pouring into the Atlantic, and not one considerable stream 

 entering the Pacific. More limited seas, which receive vast quan- 

 tities of solutions are the Caribbean and Gulf of Mexico, Arabian 

 Sea, Bay of Bengal and Yellow Sea. 



At the mouths of great rivers there exist several conditions 

 which influence the solubility and distribution of lime in the 

 adjacent seas ; these are: 1st, the amount of lime in solution 

 in the river water ; 2d, chemical reactions between substances 

 in fresh and salt water; 3d, the relative solubility of lime in 

 fresh and salt water ; 4th, the conditions of evaporation and agi- 

 tation of the brackish water ; 5th, the effects of currents. 



The proportion of solids in solution in a river is dependent 

 not only on the extent and slopes of its basin, but also on the 

 nature of the rocks exposed, and the influence of climate on 

 decay. Under like topographic conditions, silicious schists and 

 a cold climate probably yield a minimum contribution ; crystal- 

 line rocks and a warm, moist climate yield more ; limestone areas, 

 though resistant in a dry climate, suffer most rapid degradation 

 under a humid atmosphere, and the deposits of the later geo- 

 logic periods, including as they often do quantities of soluble 

 salts, charge the drainage most strongly. The following 

 analyses present specific contrasts, traceable to these geologic 

 and climatic conditions. Each analysis represents but one phase 

 of composition, which varies in each river with high and low 



