WHITES OUTLINE OF CRITERIA OO / 



"(g) The development of subaerial roots in many of the types. 



"6. A circumstance that may be observed in most coal fields in proof of 

 'abundant rainfall at the time of coal formation is the continuity of many coal 

 benches or strata from one hollow or pan over the intervening shoal or sand 

 bar into the next pan or along the slight gradients of the baselevels, a circum- 

 stance impossible except with sufficient rainfall to saturate the vegetal cover 

 and maintain a ground-water table of retarded drainage held by the obstruct- 

 ing vegetation. 



"7. Two other interesting lines of evidence for the warm climate of the 

 Carboniferous are seen, as pointed out by Potouie, in (a) the development of 

 more flowers and fruits on the lower parts of stems and branches, as in Ulo- 

 dendron, Sigillaria, and many Calamarite, a characteristic of dense tropical 

 forests at the present time, and (&) the presence in many ferns of Aphlebi* 

 which today are unknown except in tropical types." 



Steps in Study of climatic Criteria 



Although the foregoing account was clraAvn up especially to cover con- 

 ditions during the deposition of the great coal deposits, and more par- 

 ticularl}^ the Paleozoic coals, it nevertheless applies with approximately 

 equal force to all horizons. As we ascend in the geologic column, plants 

 become increasingly similar to existing plants until, in the late Pleisto- 

 cene, they largely merge with the present flora. When, for example, a 

 flora in the Mesozoic or early Cenozoic is interrogated as to its probable 

 bearing on the climatic conditions under which it grew, the first step is 

 the identification — as complete as possible — of the elements comprising 

 this flora. It is then compared as closely as possible with its obviously 

 nearest living relations, and from a study of the moisture and tempera- 

 ture requirements of these living analogues a conclusion is reached as to 

 the |)robably similar demand of the ancestral forms. If this judgment 

 was based on only one or two forms, the possibility for erroneous inter- 

 pretation would be increased; but when there are a considerable number 

 all pointing in the same direction, the probability of falling into error is 

 certainly reduced if not eliminated. 



Examples of the Application of climatic Criteria 



A number of examples may be cited. The Jurassic flora enjoyed in 

 many respects the most marvelous distribution of any known flora, either 

 living or fossil. It is known to range from Franz Josef Land, 82° north, 

 to Hope Bay, Graham Land, 63° south, and from extreme western Alaska 

 entirely around the earth to eastern Australia, or tlirough more than 155 

 degrees of latitude and more than 230 degrees of longitude. Throughout 

 this vast, practically world-wide, area there is a remarkable uniformity 

 of distribution — that is, not only in individual species widely' spread, but 



