SCIENCE 



[Vol. LV, No. 1413 



climate there, in. one of the early Tertiary 

 epochs. Unfortunately, many formations con- 

 tain no fossils or only such as have no clear 

 significance. 



More recently the nature of the sedimentary 

 rocks themselves has been recognized to be 

 indicative of climate. It was pointed out long 

 ago that ancient glacial beds in India showed 

 a period of cold climate in a region which is 

 now warm. Beds of rock salt in the Permian 

 strata of Germany were correctly interpreted 

 to mean arid conditions. But it is only re- 

 cently that the climatic relations of the less 

 extreme, through much more common, types of 

 sediments have come to be understood in a 

 general way. In the not distant future we 

 may expect to read the climatic significance not 

 only of all terrestial deposits but to some 

 extent even of the marine formations, although 

 the latter are much less affected by changes of 

 climate. 



An individual rock formation is itself a more 

 or less legible record of the local climatic con- 

 ditions at the time and place of its origin. 

 Such local climates must have been related in 

 considerable measure to temporary features of 

 the land topography. Thus, some of the de- 

 posits which indicate aridity were probably 

 laid down on the lee sides of high mountain 

 ranges. Others were due to the prevailing 

 distribution of oceans and currents. It has 

 long been recognized that the broad expansion 

 of seas in the Ordovician and Silurian periods 

 must have favored an untrammelled circula- 

 tion of currents capable of equalizing tempera- 

 tures between the tropical and polar regions 

 far more effectively than is possible to-day. 



In spite of such local influences on climate, 

 we may, by considering the climatic implica- 

 tions of all the formations of a particular 

 epoch, gain a general impression of the aver- 

 age climate of the globe for that time, and 

 these average climates may be compared among 

 themselves and with that of the present. 

 Although only a good beginning of this work 

 has been made thus far, it should be but a few 

 years till we shall understand ancient climates 

 at least as well as we already know ancient 

 land configuration. 



All that we have learned thus far about the 

 earth's climatic liistory goes to show that, ever 



since the earliest periods of which we have a 

 sufficient record, there have been frequent 

 oscillations between colder and warmer and 

 between moister and drier climates, but always 

 within narrow limits. At no time does the 

 climate appear to have been inimical to life 

 over all or even half of the globe. To-day 

 over a considerable part of the land surface 

 it is so dry, as in central Arabia, that scarcelj' 

 any plants or animals can exist. On the 

 Antarctic continent — once inhabited by a 

 varied assemblage of plants and doubtless ani- 

 mals — life has been practically exterminated 

 by the outward growth of the great ice-sheet. 

 We have no evidence in geologic history that 

 conditions have ever been especially more 

 severe; and yet in the records of almost every 

 geologic period back to the Archean we find 

 some places which have been glacial and others 

 which have been arid. 



Temperatures between 0° and 80° Centigrade, 

 at least during the growing season, are required 

 by nearly all living creatui'es, and for the vast 

 majority of them the range is less than 40°. 

 The polar districts are to-day below the mini- 

 mum, and much larger areas have been as 

 cold in certain past epochs; but we have no 

 evidence that the maximum has ever been ex- 

 ceeded. This is a very narrow range, and yet 

 the fluctuations of terrestrial climates seem to 

 have been confined within it ever since the 

 Archean period, if not earlier. The writers 

 of the older text-books of geology, placing 

 perhaps too much confldence in the nebular 

 hypothesis of Buffon and La Place, supposed 

 that the temperature of the earth had con- 

 stantly decreased from the white heat of an 

 astral period down through an era of molten 

 rock, and a later one of vaporous oceans, to 

 conditions as we find them to-day. The accu- 

 mulating testimony of the sedimentary strata 

 does not appear to support this supposition. 

 This record, extending back perhaps some hun- 

 dreds of millions of years, seems to reveal to 

 us a history marked by slight oscillations of 

 climate, but in a broad way singularly uniform 

 and without evidence of a progressive increase 

 or decrease of either general humidity or tem- 

 perature. 



In view of opinions which were generally 

 current a generation ago that geologic liistory 



