CLIMATE AND TERRESTRIAL DEPOSITS 295 



to an exteat, depending upon the size of these factors, the climatic 

 indications given by erosion. In proportion as such occur, however, 

 the climatic influences in the region of deposition grow more pro- 

 nounced, until finally over the larger deltas, where the fine-grained 

 waste is deposited on nearly horizontal surfaces, the climatic effects 

 become a dominant influence in the nature of sedimentation. 



From the extremes of cool and rainy climates on the one hand to 

 hot and arid on the other a gradation in character of deposits may be 

 observed corresponding to that gradation which exists in climatic 

 cause. For the sake of discussion, however, artificial division lines 

 must be drawn separating the climates into several categories, as has 

 been done. In consequence, except in clearly marked instances there 

 may be some doubt as to the exact climatic division to which a certain 

 deposit belongs. To reach the greatest certainty every line of evi- 

 dence must be followed to its end and given appropriate weight in 

 governing the conclusion. Only by the convergence of many proba- 

 bilities may reasonable certainty be attained. 



The most obvious chemical features of subaerial river deposits 

 which depend upon the climatic gamut consist in the antithetical 

 relations of carbon and ferric oxide. Supplemental to these two chief 

 indicators stand the other more or less soluble substances, magnesium 

 and calcium carbonates, hydrous calcium sulphate, sodium and 

 potassium chlorides; wholly absent from river deposits of the one 

 climatic extreme, appearing in successive order in deposits made 

 under climates representing gradations toward and to the other. 

 Supplemental to these primary chemical distinctions are the textural, 

 structural, and organic evidences. 



Varying powers of erosion and transportation giving rise to varying 

 quality and quantity of sediment are seen to be the most delicate 

 stratigraphic indicators of climatic -fluctuations. On the other hand 

 the chemical and organic conditions accompanying the deposition 

 of the sediment upon the delta plain are the more secure indicators of 

 the stable and average climatic conditions under which the formation 

 as a whole was made. 



{To he concluded) 



