FUTUEE HABITABILITY OF THE EAETH CHAMBEELIN. 379 



perature at the surface of the earth seems none the less to be inti- 

 mately dependent on the atmosphere and to constitute a further index 

 of its critical character. 



To appreciate the full significance of so effective a control of life 

 conditions poised thus between excess and deficiency, with the danger 

 line close on either hand, while the possibilities were so free and so 

 wide, there is need for some measure of the time through which the 

 delicate poise has been held. But there are now no means for any 

 close measure of the geologic ages ; there are merely rough estimates 

 which give the order of magnitude. Life was far advjyiced in its 

 career when first a readable record was made; but yet, since that 

 record began, 100,000 feet of sediments at least — not to choose the 

 largest estimates — have been laid down by the slow methods of wash 

 from the land and lodgment in the basins. The number of years this 

 implies has been placed variously from 50,000,000 to 100,000,000, 

 with, indeed, higher figures as well as lower. Merely to roughly 

 scale the order of magnitude without pretense of accuracy, let us take 

 the midway figure of 75,000,000 years as representative. Let this be 

 divided into 15 periods which may be made to average 5,000,000 years 

 each, and these will roughly represent the technical " periods " of 

 geologists. By this rough scale we may space out such of the great 

 events as we need now to review. These events are such as tell us of 

 the states of the atmosphere and of the temperatures that prevailed 

 on the surface of the earth at a sufficient number of the periods to 

 show the general tenor of past history in matters critical to life. 



As an index of arid conditions we naturally turn to the products 

 of evaporation. In interpreting there is need to note that there may 

 be small excesses of evaporation over precipitation without giving 

 rise to appreciable deposits of evaporation products, for in almost 

 all cases the area that collects rainfall is larger than the portion of 

 the basin that actually holds it, because some point on the rim of the 

 basin is almost inevitably lower than the rest, and this lowest point 

 permits the accumulating waters to drain off to its level, so that it 

 is only the smaller water surface thus left that is exposed to contin- 

 uous evaporation and takes part in the concentration of dissolved sub- 

 stances into beds of solid salt and g^'-psum. It is therefore fairly safe 

 to infer a decidedly arid climate when beds of salt and gypsum are 

 found spread over wide areas, especially if these also bear appro- 

 priate physical characteristics and if the adjacent deposits are totally 

 free of life or carry only fossils of such types of life as can tolerate 

 a high degree of salinity, or such as show signs of depauperization 

 by the adverse conditions of aridity and salinity. 



Now, extensive deposits of salt and gypsum are found in the Salt 

 Range of India in strata of the Cambrian period, the earliest of the 

 15 periods that make up our rough scale of 75,000,000 years. Because 



