550 F. H. KNOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



plants now seem to tolerate strong sunlight, it is possible, even probable, 

 that through exposure to strong light they have come to be greater light- 

 demanders. The ones grown under the postulated cloud conditions oT 

 the earlier geologic ages may have corresponded more nearly to our 

 present shade-tolerant forms. 



In the living flora the ferns hold a subordinate position, though thera 

 are perhaps as many specific types now as there were at any time in the 

 past. The living ferns are preeminently shade-loving plants; in fact, 

 few of them can long survive exposure to strong sunlight. Is it not 

 possible that their shade-loving habit is a direct inheritance from the 

 early geologic ages, when they were developed under a practically con- 

 tinuous cloud mantle? Ferns have now essentially the same structure, 

 method of reproduction, and ecological requirements that they had 

 throughout the Paleozoic and Mesozoic. If the ferns of the earlier ages 

 were developed under conditions of strong sunlight, they have completelv 

 changed their habit without this fact being reflected in the structure. 



OBSTACLES TO THE ACCEPTANCE OF EARTH HEAT 



Other supposedly serious obstacles to the acceptance of this hypothesis 

 have recently been formulated by Barrell.^^ He holds that "the amount 

 of radioactivity known to exist in' the crust appears to be adequate, or 

 more than adequate, to account for the whole emanation of heat," but 

 his deduction from this, namely, that "there is no evidence, therefore, 

 that the earth is cooling and that the crust gave forth more heat in 

 earlier times," does not seem clear. Under the present system of solar 

 control, there may be a compensatory adjustment of heat derived from 

 the two sources (earth and sun) that was lacking when the sun was not 

 the dominant factor it now is. To my mind, logic supports the conten- 

 tion of a cooling globe during the major part of geologic history. 



Barren's second point — namely, that the deeper parts of many pre- 

 Cambrian formations, such as the Unkar and Chuar, exposed in the 

 depths of the Grand Can3^on section, do not show regional metamorphism, 

 and hence that the "temperature gradient since the late pre-Cambrian 

 could not, therefore, have been notably higher than at present" — appears 

 to be based on the supposition that the entire section of perhaps 20,000 

 feet was buried and remained at a great depth for ages, while as a matter 

 of fact it probably came to the surface in late x^lgonkian time and 

 remained at or near the surface for a very long time, as shown by the 



^Joseph Barrel! : Rhythms and the measurement of geologic time. Bull. Geol. Soc. 

 Am., vol. 28, 1917, pp. 901, 902. 



