902 J. BAKRKLL MEASUKEMENT8 OF GEOLOGIC TIME 



kSecoiul, tlie deeper parts of many Precambriaii formations^ such as the 

 Uiikar and Chuar exposed in the depths of the Grand Canyon of Arizona., 

 have been buried to a depth of several miles in the late Precambrian, and 

 again in the Paleozoic, and yet show no regional metamorphism, com- 

 paring in this respect with Paleozoic and Mesozoic formations. The tem- 

 ])erature gradient since the late Precambrian could not, therefore, have 

 been notably higher than the present. 



Third, the quantity of heat wliich the earth delivers to the atmosphere 

 is now and must always have been inconsequential in comparison witli 

 that derived at present from the sun. For instance, to have conducted 

 five times the heat, the temperature gradient would have to be five times 

 as steep, giving a temperature of molten rocks at a depth of five miles. 



Fourth, the presence of banding in certain argillites of early Precam- 

 brian times in Norway and Canada, associated w^ith ancient glacial de- 

 posits, is directly comparable with the annual banding in stratification in 

 Pleistocene clays in those same regions, and testifies to the dependence 

 of temperature in those times on solar radiation with an atmospheric con- 

 dition which permitted the existence of winter. 



Fifth, the abundance of carbon in the clays of early Precambrian time 

 suggests the presence of sunlight sufficient to carry forward photosyn- 

 thesis in plants and therefore the absence of an extremely dense cloud 

 envelope. 



Sixth, glacial conditions are found to be associated with times of ter- 

 restrial revolutions closing the eras and to have recurred at times since 

 the Middle Precambrian. Similarly, periods of wide-spread aridity have 

 recurred at intervals since the Upper Precambrian. The composition of 

 the atmosphere and ranges of temperature have, therefore, been subject 

 to fluctuation through all geological time and have not shown a steady 

 decline, formerly suggested as a cause for Pleistocene glaciation. The 

 normal existence of warm temperate climates in high latitudes and the 

 Permian glaciation in low latitudes must therefore rest on some other 

 explanation. 



Consequently, we must return to the conclusion that the sun through 

 all geological time has been the only effective source of terrestrial warmth. 

 But life began to evolve in the earliest known times, and since then the 

 temperature limits compatible with the continued existence of the various 

 classes of organisms have never been exceeded. When it is considered 

 how narrow these limits are, as compared with the absolute cold of space 

 on the one hand and the temperature of the sun on the other, it is seen 

 that not only has the sun radiated energy through all geological time, 

 but that the flow of energy, although subject to rhythmic fluctuations, 



