504 F. H. KXOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



venture to quote it entire. Although it was drawn up especially to cover 

 the conditions during the deposition of the great coal deposits, and more 

 particularly the Paleozoic coals, it nevertheless applies with equal force 

 to all horizons. White* writes as follows : 



"During the times of deposition of most of ttie principal coal groups the 

 climate has been characterized by (1) general mildness of temperature, ap- 

 proaching in most cases tropical or subtropical ; (2) conspicuous equability or 

 approximation to uniformity of climatic conditions, which, with a few excep- 

 tions, appear to have lacked cold winters or severe frosts; (3) a generally 

 high humidity, the rainfall being from, moderately heavy to very heavy and 

 fairly well distributed through the year, though in many cases there is evi- 

 dence of the occurrence of dry periods, which, however, seem ordinarily to 

 have been comparatively short and not severe; (4) an amazingly wide geo- 

 graphical distribution of these genial and equable climates, which occurred 

 seemingly in almost uniform development simultaneously in the high and in 

 the low latitudes of both the Northern and the Southern Hemispheres. This 

 shows either that the essentially uniform climatic conditions were truly ex- 

 traordinary in geographic extent, with little regard to modern climatic zones, 

 or that the formation of coal was mainly confined to the areas of the above- 

 prescribed climatic environment. 



"The principal criteria as to climate offered by the fossil plant remains pre- 

 served either in the coal or in the enveloping shales and sandstones and, serv- 

 ing as a basis for the conclusions stated above, may be summarized as follows : 



"1. Relative abundance or luxuriance and large size of terrestrial vegeta- 

 tion — that is, rankness of growth — indicating favorable conditions of tempera- 

 ture, humidity, etc. 



"2. Character, condition, and amormt of the land-plant material preserved 

 as coal or carbonized in the rocks. The formation of xyloid coal of the ordi- 

 nary types, composed mainly of subaerial vascular plant remains, indicates 

 humidity. In regions of cool temperature the humidity required for the for- 

 mation of peat — the initial state of coal — -is moderate; in warmer climates, 

 where decay is more rapid, not only must the humidity be greatly increased 

 in order to provide the necessary wetness to retard decomposition, but there 

 must be no long dry season of the year for the too great reduction of the 

 water cover. The observations of peat formation at the present day in trop- 

 ical climates show that in order to permit the deposition of peat the rainfall 

 must be both very heavy and fairly well distributed through the entire year. 



"3. Great radial distribution, seemingly over the greater part of the earth, 

 and especially over wide ranges of latitude, of identical species and genera in 

 characteristic association, indicating the extension of approximately uniform 

 climatic conditions in these regions. Floras identical, or essentially identical, 

 in remote or detached regions can owe their identity to no other cause than 

 approximate continuity of the environment, whether that continuity is geo- 

 graphic or chronographic. Conversely, migration of a flora without change is 

 possible only through regions of essentially identical environmental conditions. 

 Illustrations are found in the Carboniferous, Triassic, Jurassic, and Lower 



* David White : The origin of coal. U. S. Bureau of Mines, Bull. 38, 1913. p. 6'; 



