J CENTURY OF GEOLOGY. 551 



But it would be strange, indeed, if the cold of tlie Glacial epoch 

 should be absolutely unique. Attention was soon called to similar 

 marks in rocks of other geological periods, especially in the Per- 

 mian of the southern hemisphere. This opened up the general 

 question of geological climates and their causes. 



Perhaps no subject connected with the physics of the earth is 

 more obscure and difficult than this. The facts, as far as we know 

 them, are briefly as follows: (1) All the evidence we have point to 

 a high, even an ultra-tropical, climate in early geological times; 

 (2) all the evidence points to a uniform distribution of this early 

 high temperature, so that the zonal arrangement of temperatures, 

 such as characterizes present climates, did not then exist; (3) tem- 

 perature zones were apparently first introduced in the late Meso- 

 zoic (Cretaceous) or early Tertiary times, and during the Tertiary 

 the colder zones were successively added, until at the end there 

 was formed a polar ice-cap as now. 



Thus far all might be explained by progressive cooling of the 

 earth and progressive clearing of the atmosphere of its excess CO2 

 and aqueous vapor. But (4) from time to time (i. e,, at critical 

 periods) there occurred great oscillations of temperature, the last 

 and probably the greatest of these being the Glacial period. The 

 cause of these great oscillations of temperature, and especially the 

 cause of the glacial climate, is one of the most interesting and yet 

 one of the obscurest and therefore one of most hotly disputed points 

 in geology. Indeed, the subject has entered into the region of 

 almost profitless discussion. "We must wait for further light and 

 for another century. Only one remark seems called for here. 

 It is in accordance with a true scientific method that we should 

 exhaust terrestrial causes before we resort to cosmical. The most 

 usual terrestrial cause invoked is the oscillation of the earth's crust. 

 But recently Chamberlin, in a most suggestive paper,* has in- 

 voked oscillations in the composition of the atmosphere, especially 

 in its proportion of CO2, as the immediate cause, although this in 

 turn is due to oscillations of the earth's crust. 



THE NEW GEOLOGY. 



Heretofore the geological history of the earth has been studied 

 only in the record of stratified rocks and their contained fossils. 

 But in every place there have been land-periods in which, of 

 course, erosion took the place of sedimentation. This kind of 

 record is very imperfect, because there are no fossils. Until re- 

 cently no account was taken of these erosion-periods except as 



* Journal of Geology, vol. vi, p. 597, 1898, and vol. vii, p. 545, 1899. 



