920 e. blackwelder characteristics of chemical deposits 



Climatic Types of Sediments 



classification 



It will serve present purposes to divide all continental sediments into 

 three groups : 



(a) the pure or extreme types, 



(h) the mixed types, 



(c) the intermediate types. 



PURE OR EXTREME TYPES 



Sub-Arctic type. — In the first group we may distinguish the sub-Arctic, 

 the humid tropical, and the desert types; in the second (or mixed) group 

 deposits made under alternating wet and dry seasons, and also sediments 

 that have been transferred from one climate to another, and in the last 

 (or intermediate) group various inter gradations between the primary 

 types. For the sake of brevity, I omit the purely glacial Arctic type. 



In the sub- Arctic regions the deposits made by rivers in their deltas 

 and structural depressions are probably the most important in quantity. 

 The ideal conditions are found in the valleys of the Yukon, Mackenzie, 

 Lena, Yenesei, and many others. In these fluviatile sediments the min- 

 erals are but little decayed, because frost action is the chief process of 

 weathering at the source. They are also associated with abundant car- 

 bonized vegetable matter, because vegetation is not only plentiful, but 

 under the prevailing conditions decays slowly and leaves a copious residue. 

 The result is the deposition of complex anhydrous silts, undecayed wackes, 

 and gravelsf in which some shade of gray or black is the characteristic 

 color, for a saturated and often a frozen soil charged with decaying or- 

 ganic matter leaves little opportunity for oxidation and the development 

 of the brighter colors. For the same reason sun-cracks rarely develop. 

 Carbonized wood is not uncommon in such deposits, and leaves may 

 locally be abundant ; but vertebrates are rare and mollusks much more so. 



The associated minor deposits, any one of which may locally become 

 predominant, are peat, glacial till, lacustrine silts, soil flows, and talus. 

 The whole series is characterized by the abundance of undecayed silicate 

 minerals, sombre colors, by absence of saline deposits, and l)y absence or 

 scarcity of other formations tliat are distinctive of the hot climates. 



Humid, tropical type. — In partial contrast to the sub-Arctic is the 

 humid, tropical type, best exemplified by deposits made on the windward 

 side of tropical islands and coasts. Obviously, it is far less extensive in 

 area, and therefore in quantitative importance, than the sub-Arctic series, 



