924 E. BLACKWELDER CHARACTERISTICS OF CHEMICAL DEPOSITS 



loess deposits^ but this has been definitely established in very few eases. 

 Some of the greatest loess deposits seem to have no relation to glaciation. 



An increase in the occasional rainfall of the temperate regions to forty 

 or more inches a year produces in the sediments changes important 

 enough to deserve recognition. In these subhumid temperate regions the 

 rivers, fresh lakes, and marshes are the chief agencies of sedimentation. 

 The alluvium of the Tennessee Eiver and of many other short rivers in 

 tlie temperate zone would serve as examples. We must bear in mind that 

 nearly all of the streams in regions of this kind have reached grade (pro- 

 file of equilibrium), and hence make deposits of noteworthy thickness 

 only at their mouths or where they cross structural depressions, such as 

 grabens and down warps. Both gravels on the one hand and ferric clays 

 on the other are deficient. Sandy silts and clay silts predominate. Even 

 if derived from red soils, after deposition they normally turn gray or 

 grayish brown because of the generous amount of residual organic matter 

 present. Black peaty deposits are rather common, but the formation of 

 typical loess is not favored. Neither till nor saline deposits can be made 

 under such conditions. Like the deposits of the tropical moist belt, these 

 sediments have generally lost by leaching most of the more soluble con- 

 stituents, such as the alkalis, lime, and magnesia, and yet they do not 

 contain a high percentage of the aluminous hydroxides which are distinc- 

 tive of the humid tropics. 



Limitations of space prevent me from discussing even casually the 

 many other interesting mixed and intermediate types, such as the hybrids 

 between the sub-Arctic and the desert (western Siberia), the Alpine sub- 

 Arctic with the tropical (central Andes, Euwenzori, etcetera), or with 

 the type of alternating climates (sub-Himalayan). Most of these still 

 await critical study, although many of their characteristics might safely 

 be predicted in advance by reasoning deductively from known principles. 



Need of Exact Data 



In closing, I wish to express the opinion that the present study of sedi- 

 mentation rests too largely on such deductive reasoning from physiog- 

 raphy and climate and too little on the rigorous inductive consideration 

 of minute field observations and laboratory experiments. The field geol- 

 ogist can render very important service by giving as close attention as his 

 time may permit to all modern sedimentary deposits which he finds and 

 to their mode and conditions of origin. By collecting samples of such 

 sediments with adequate field notes and afterward submitting them to 

 specialists for study, he will supply the material for much valuable labora- 

 tory research and will thus help to accelerate the genuine advance of our 

 knowledge of the sediments. 



