430 GEOLOGY. 



for glaciation, certain of the agencies involved became dominant and 

 tended to intensify and accelerate glaciation for a time, until they 

 either pushed the effects to an extreme from which a reaction was 

 inevitable, or they exhausted themselves temporarily, while other 

 agencies of opposite phase, which had been subordinate until then, 

 became dominant and forced a reaction. 



When a reaction was set up, it in like manner was pushed to an 

 extreme, and deglaciation extended beyond the point of equilibrium 

 for the average conditions. And so oscillations beyond and short 

 of the mean state, gave a rhythmical phase to the glaciation of the 

 period. The rhythm, we learn from observation, took the form of 

 a series of sub-equal oscillations with declining time-intervals. There 

 seem to have been no great differences in the amplitude of the ice 

 advances. Observation does not permit us to speak as confidently 

 of the extents of the recessions. It is important to note that the fun- 

 damental or general conditions remained effective throughout the 

 period, and that the oscillations are regarded only as rhythms super- 

 posed on these general conditions. The more intense phases of these 

 rhythms were, however, the only portions of the series that recorded 

 themselves in glaciation near the borders of the glaciated areas, and 

 were perhaps the only portions that recorded themselves in continental 

 glaciations at all. The retrocessional phases may have been recorded 

 only in cool climates m high latitudes, and in glaciation at high alti- 

 tudes. 



Among the specially intensifying agencies that are thought to 

 have pushed glaciation to its climaxes, the following are recognized: 



1. The higher carbonation of the ocean necessary to bring its car- 

 bon dioxide into equilibrium with that of the atmosphere at the 

 lower temperature that had been induced by the general conditions, 

 especially in the high latitudes. This lower temperature of the water 

 gave the sea a higher coefficient of absorption of carbon dioxide. (Sec 

 previous discussion under Permian, Vol. II, p. 665.) 



2. A special process of super-carbonation of the ocean through 

 the agency of freezing in high latitudes, which cooperated with the 

 above. 



3. A reduction of the organic extraction of lime and the other bases 

 of the bicarbonates, which otherwise would have freed carbon dioxide. 



4. An increased reflection from the snow-fields and hence a reduced 



