CLIMATE AND TERRESTRIAL DEPOSITS 269 



able potash and magnesia. From this would be inferred an origin, 

 under cool climatic conditions, in line with the inference previously 

 drawn from the Carboniferous conglomerates of Rhode Island — 

 calling attention to the importance of microscopical or chemical 

 examination and comparison of argillaceous sediments of similar 

 continental but presumably of unlike climatic origin. In contrast 

 with the muscovite shales of the coal-measures may be noted the 

 absence of muscovite in the carbonaceous shales of the Hudson River 

 and Hamilton periods which underlie the Carboniferous and have 

 consequently been subjected to equal or even greater metamorphism. 

 W^iile these latter shales are of marine origin it is not clear that that 

 fact alone could lead to this peculiar distinction. 



That a cool climate, while undoubtedly favorable, is not necessary 

 for the production of. coal-measures is, however, shown at the present 

 time by the swamps of the Amazon and, in the past, by the warm- 

 temperate flora of the Eocene coals of the Pacific slope. The absence 

 of frost rather than a hot climate is, however, all that is necessarily 

 implied by the Eocene vegetation. 



EFFECTS OF INTERMITTENTLY RAINY CLIMATES 



Intermediate character of deposits. — Climates of this class are 

 such as characterize those portions of the world where crops may be 

 grown without the aid of irrigation, but where one or more months 

 may be relatively free from rain. Under these familiar conditions 

 the soil of the flood plains normally contains considerable humus, 

 but much of it is yellow or red, instead of brown or black, from the 

 subordination in quantity of the humus to the ferric hydrate. The 

 greater part of the soil of flood plains is sufficiently dry during a grow- 

 ing season for such crops as corn and cotton. The clays are slightly 

 calcareous and occasionally sufficiently so to give rise to the so-called 

 "buckshot" soils such as are found over portions of the Mississippi 

 flood plain. ^ The subsoils of such plains are observed to carry more 

 compact clay and less humus than the soil, the carbon thus gradually 

 disappearing with depth in aerated soils. 



Only in the lower bottoms or abandoned ox bows is the land so 

 saturated with moisture that swamp vegetation dominates, organic 



I E. W. Hilgard, Soils in the Humid and Arid Regions, igo6, p. 116. 



