294 STUDIES FOR STUDENTS 



tance of this reaction lies in the fact that the blue or green tint, wher- 

 ever it occurs, indicates a lack of aeration, usually by the stagnation 

 of water, in consequence of imperfect drainage.^ 



It is seen in conclusion that where the light color is due to mechani- 

 cal causes the mode of origin of the formation and the climatic con- 

 ditions must be determined on other grounds than color. Further, 

 where limited development of white, gray, green, or blue occurs, owing 

 to chemical action upon the iron, the conditions represent a variable 

 balance between the action of iron and carbon determined by local 

 topographic rather than climatic conditions. Where such colors are 

 rather abundant, however, over broad areas of non-marine formations 

 a mean, rather than an extreme, climate is indicated. 



The origin of gray to black formations. — The conclusions on this 

 topic are rather obvious from the preceding discussion on the "effects 

 ofjconstantly rainy climates." They need, therefore, only be sum- 

 marized at this point for completness. 



Where a whole formation, representing an ancient fioodplain or 

 delta, shows in its unweathered portions an absence throughout of 

 the colors due to iron oxide, and a variable presence of carbon, giving 

 grays to black, the inference is that the formation accumulated under 

 a continuously rainy climate or one which in the drier season was 

 sui^ciently cool or cold to prevent noteworthy evaporation; such 

 climates as exist in Ireland, Iceland, or western Alaska; to a minor 

 extent on windward slopes in the trade-wind zones and also to a minor 

 extent in a few tropical belts which never quite escape from the shifting 

 zones of tropical rains. 



CONCLUSIONS ON CLIMATIC INFLUENCES IN REGIONS 

 OF DEPOSITION 



It was seen in the discussion on the relations of climate to regions 

 of erosion that climate was one of the controlling factors in deter- 

 mining the quantity, but more especially the physical and chemical 

 nature, of the sediment supplied to the rivers. But some large river 

 systems have their sources in climatic zones distinct from that of their 

 lower courses. Furthermore, the comminution and decay involved 

 in transportation and the varied contributions of tributaries obscure 



I Hilgard, Soils, 1906, p. 45. 



