CLIMATES OF GEOLOGIC TIME. 275 



and that when the dust is least, other things being equal, the output of energy will be greatest. Not 

 only may the intensity of the emitted radiation vary because of changes in the transparency of the 

 solar atmosphere but also because of any variations in the temperature of the effective solar surface 

 which, it would seem, might well be hottest when most agitated, or at the times of spot maxima, and 

 coolest when most quiescent, or at the times of spot minima" (b: 16). 



BIOLOGIC EVIDENCE. 



In the previous pages there has been presented the evidence for cold climates during 

 geologic time as furnished by the presence of the various tillites. This presentation has also 

 been made from the standpoint of discovery of the tilUtes, wliich in general is in harmony 

 with geologic chi-onology, i. e., the youngest tillites were the first to be observed, while the 

 most ancient one has been discovered recently. 



Variability of climate is also to be observed in the succession of plants and animals as 

 recorded in the fossils of the sedimentary rocks. In this study we are guided by the 

 distribution of living organisms and the postulate that temperature conditions have always 

 operated very much as they do now upon the Uving things of the land and waters. In 

 presenting this biologic evidence we shall, however, begin at the beginning of geologic time 

 and trace it to modern days, for the reason that Ufe has constantly varied and evolved from 

 the more simple to the more complex organisms. 



Proterozoic— The first era known to us wdth sedimentary formations that are not 

 greatly altered is the Proterozoic, a time of enormous duration, so long indeed that some 

 geologists do not hesitate to say that it endui-ed as long as all subsequent time. These rocks 

 are best known and occur most extensively over the southern half of the great area of 

 2,000,000 square miles covered by the Canadian shield. There were at least four cycles of 

 rock-making, each one of which, in the area just north of the Great Lakes and the St. 

 Lawrence River, was separated from the next by a period of mountain-making. These 

 mountains were domed or batholithic masses of vertical uplift due to vast bodies of deep- 

 seated granitic magmas rising beneath and into the sediments. In the Grenville area of 

 Canada, Adams and Barlow (1910) tell us that the total thickness of the pre-Proterozoic 

 rocks alone is 94,406 feet, or nearly 18 miles. Of this vast mass more than half (50,286 feet) 

 is either pure limestone, magnesian Imiestone, or dolomite, and single beds are known with a 

 thickness of 1,500 feet. Certainly so much hmestone represents not only a vast duration of 

 time but also warm waters teeming with life, almost nothing of which is as yet known. There 

 is further evidence of life in the widely distributed graphites, carbon derived from plants 

 and animals, which make up from 3 to 10 per cent by weight of the rocks of the Adirondacks 

 (Bastin, 1910). The graphite occurs in beds up to 13 feet thick, and at Olonetz, Finland, 

 there is an anthracite bed 7 feet thick. 



It is also becoming plain that there was in the Proterozoic a very great amount of 

 fresh-water and subaerial deposits, the so-called continental deposits, some of which indicate 

 arid chmates. Because of the apparent dominance of continental deposits and the great 

 scarcity of organic remains throughout the Proterozoic, Walcott has called this time the 

 Lipalian era (1910: 14). 



We have seen that the Proterozoic began with a glacial period, as evidenced by the 

 tillites of Canada, but that this frigid condition did not last long is attested by the younger 

 Lower Huronian limestones of Steeprock Lake, Ontario, havmg a thickness of from 500 to 

 700 feet and replete with Arch^ocyathinse, coral-like animals up to 15 inches in diameter, 

 and forming reef limestones several feet thick, found there by Lawson and described by 

 Walcott (1912). This discovery is of the greatest value, and opens out a new field for 

 paleontologic endeavor in Proterozoic strata and for philosophic speculation as to the 

 time and conditions when Ufe originated. 



