362 A. p. COLEMAN GLAClAt PERIODS ANt) GEOLOGICAL THEORIES 



removal of the ice with a warmer climate than at present, followed by the 

 formation of another great ice-sheet. Similar changes occurred in the 

 Alps and Eussia, and, as many geologists believe, in northwestern Europe 

 also ; and two widely separated ice-advances are reported from Patagonia, 

 the Andes, and mount Kosciusko in Australia. It is probable that at 

 least one interglacial period was worldwide. 



Conclusions 



From the previous discussion certain conclusions may be drawn : 



1. Glacial periods occur at wide intervals almost to the beginning of 

 known geological time and are a normal feature of the earth's history. 



2. Glacial deposits were so widely spread in at least three of the periods 

 that the temperate and polar regions of both hemispheres were affected, 

 and in one of them the tropics also were invaded by land ice which 

 reached sealevel on three continents. 



3. There is a remarkable focussing of great ice-sheets about certain 

 centers in two of the glacial periods, the north Atlantic in the Pleistocene 

 and the Indian ocean in the Permo-Carboniferous. 



4. At least one interglacial period was probably worldwide in the 

 Pleistocene, and important interglacial beds occur in the older boulder 

 clay deposits also. It is necessary, then, to account for very long periods 

 of mild climate undergoing only slow change, separated by comparatively 

 short periods of rapidly alternating cold and warm climate. 



If the foregoing conclusions from glacial geology are admitted, various 

 theoretical consequences ensue. If ice ages go back almost to the earliest 

 known times, we must give up the idea of a formerly molten earth slowly 

 cooling from age to age, unless the cooling ran its course before the 

 earliest geological formations were deposited. The nebular theory, as 

 usually understood, must be relegated to a past so distant as to have little 

 interest for the geologist, or must be thrown overboard altogether. 



With the nebular theory goes also Sir George Darwin's ingenious theory 

 of the origin of the moon by tidal action, unless that, too, be pushed many 

 millions of years farther back than his calculations indicate. The cross- 

 bedded sands and the boulder clays, now changed into hard rocks in the 

 Huronian, hint of no greater heat and of no stronger tidal action then 

 than now. There was no commotion such as must have been raised in 

 the seas of an earth rotating at four times its present rate beneath a 

 moon one-half as far away as at present. 



The departure of the molten earth, with its supposedly thin and easily 

 bent or broken crust, sweeps away with it many of the usual theories of 



