WHITE S OUTLINE OF CRITERIA dOO 



CkITERIA for IaTEIU'RETATIOX of CLIMATIC CoNDlTIOXS 



In seeking to ascertain the bearing of the fossil floras on the climatic 

 conditions that obtained at the time they were living, it is desirable to set 

 forth the criteria that must form the basis of such judgment. In draw- 

 ing conclusions from individual organisms in an inquiry of this kind, 

 dependence must, of course, be placed on our knowledge of the present- 

 day requirements of similar species, and the results must always be sub- 

 ject to possibility of error from two sources : first, from the incorrect 

 placing biologically of the organism, and, second, from the fact that its 

 requirements in past geologic time may. not have been the same as those 

 which now dominate the life activities of its supposed analogue. How- 

 ever, when all or a very considerable proportion of the elements of a flora 

 appear to point in the same direction, the liability to serious error is 

 minimized if not eliminated. 



An outline of the criteria most relied on in the interpretation of cli- 

 matic conditions has been so succinctly stated by White in his paper on 

 the "Origin of coal" that I venture to quote it entire : 



"During the times of deposition of most of tlie 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 modei'ately 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 extra- 

 ordinary 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, 

 preserved either in the coal or in the enveloping shales and sandstones, and 

 serving 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, etcetera. 



"2, Character, condition, and amount 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 recpiired for the for- 

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



XXV— Bull. Geol. Soc. Am., Vol. 32, 1920 



