282 A CENTURY OF GEOLOGY. 



sheet. This idea has been eonhnned l)y all subsequent investigation, 

 especially here in America. 



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

 should be absolutely unique. Attention was soon called to similar 

 marks in rocks of other geological periods, especially in the Permian 

 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) temperature zones were apparently 

 first introduced in the late Mesozoic (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 COj and 

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

 there occurred great oscillations of temperature, the last and probabh' 

 the greatest of these being the Glacial period. The cause of these 

 great oscillations of temperature, and especially the cause of the gla- 

 cial 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 scien- 

 tific method that we should exhaust terrestrial causes before we resort 

 to cosmical. The most usual terrestrial cause invoked is the oscilla- 

 tion of the earth's crust. But recently Chamberlin, in a most sugges- 

 tive paper, ^ has invoked oscillations in the composition of the 

 atmosphere, especially in its proportion of CO,, 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 imper- 

 fect, because there are no fossils. Until recently no account was 

 taken of these erosion periods except as breaks of indefinite length in 

 the record— as lost intervals. But now, and mainly through the work 



Journal of Geology, Vol. VI, p. 597, 1898, and Vol. Vll, p. 545, 1899. 



