620 PROCEEDINGS OF THE NEW YORK MEETING 



the geologic record and are apt to be classed \yith marginal marine formations 

 on account of their lack of definite characteristics. In semiarid climates the 

 deposits show not only a higher oxidation, but the clays contain more potash 

 and are markedly calcareous. As swamp deposits diminish, mud-cracked 

 strata increase and may become noteworthy. Under arid climates potash-bear- 

 ing and calcareous clays and mud-cracked strata become more prominent ; 

 peolian and especially evaporation deposits of gypsum and salt also occur; 

 oxidation of the delta materials is almost \miversal, with the result that dark 

 or variegated shales are nearly absent. A discussion of the climatic signifi- 

 cance of color follows, and it is concluded that red is the common color of con- 

 solidated fluviatile deposits which were subjected at the time of their accumu- 

 lation to seasons of subaerial oxidation, and that red shales and sandstones 

 may therefore arise from either the yellow or red alluvium of intermittently 

 rainy, semiarid, or arid climates. In conclusion it is pointed out that while 

 the varying powers of erosion and transportation are delicate stratigraphic 

 indicatoi's of clintatie fluctuations, the chemical and organic control accom- 

 panying the deposition are the more secure indicators of the average climatic 

 conditions. 



Part III. Relations of Climate to fluviatile Transportation 



This subject is considered last, since except iu connection with the condi- 

 tions of erosion and deposition the climatic relations of transportation are not 

 conspicuous. Laboratory experiments and observations of river action lead to 

 the induction that one of the reasons why rivers carry coarse material such 

 short distances is because of the mechanical weakness of all but quartz and 

 quartzite pebbles and the consequent loading up of the current with finer 

 material. The slope of a graded river is in delicate adjustment between fhe 

 amount of water and the amount of the load, and consequently every change 

 in these factors throws it out of equilibrium. As a result, sometimes cutting 

 and sometimes depositing is in excess throughout the middle portion of its 

 course. In the case of long rivers flowing from regions in topographic youth or 

 maturity, as those flowing east from the Andes and Rockies, a very great dif- 

 ference in grade exists, according as the river course lies in a semiarid or 

 rainy climate. Under semiarid climates piedmont slopes may be built up to a 

 thickness of a thousand feet or more, and in a climatic variation to one of 

 pluvial nature this may be i-apidly carried away, and, since it consists largely 

 of sand and gravel, a great sand or conglomerate formation will be spread over 

 the delta plain. The dissection in the early Pleistocene or latest Tertiary of 

 the high plains fronting the Rocky mountains is cited as an instance. On the 

 other hand, upon a change of climate from humid to semiarid, another sand 

 and conglomerate formation will be laid down close to the mountains as a 

 piedmont slope, building up the river channel to a steeper grade. From this it 

 is concluded that the region of deposit of river gravels and its shiftings is of 

 great climatic significance. 



Summary and Conclusion 



Summing up the preceding discussion, it is to be concluded that conglomerate 

 and sandstone formations intercalated between others of different nature may 

 be due to three distinct causes : 



i 



