438 GEOLOGY, 



by a reversal of the process, in which the ocean gave forth more carbon 

 dioxide than it received. 



2. The cumulative effects of the ice-covering in reducing the car- 

 bonation of the rocks. 



When once the extension of the glaciation had been checked and a 

 retrocession begun, the following agencies are thought to have abetted 

 it, and forced it, in turn, to an extreme. 



1. The reversal of the oceanic action, by which it gave out in the 

 warm regions more carbon dioxide than it absorbed in the cold regions, 

 and thus lost its higher state of carbonation. 



2. The increase of the secretion of lime in the ocean, setting free 

 the second equivalent of carbon dioxide of the calcium bicarbonate. 

 This was due to increasing warmth of the ocean and to the spread 

 of the shallow sea-border on the land as the result of the return to 

 the ocean of. the water previously locked up in the ice, the warmth 

 acting both through dissociation and through lime-secreting organisms. 



3. An increase in the moisture in the air, and hence an increased 

 absorption and retention of solar radiation. 



4. A reduced reflection from the snow-fields, ice-clouds, and frozen 

 fogs, and the substitution of the more thermally absorbent dark earth, 

 water-clouds and fogs. 



The cumulative effects of these and some minor agencies are pre- 

 sumed to have pushed the glaciation back to a state appreciably beyond 

 that required by the average effects of the agencies involved, and 

 hence to have prepared the way for a new stage of aggressive glaciation. 

 The agencies are thought to have been competent to produce entire 

 deglaciation of the lowlands, in the longer interglacial epochs. They 

 are not thought to have been able to restore the deep oceanic circu- 

 lation to the pre-giacial state, but only to check and change the car- 

 bonating effects. 



In all this period of oscillation it is assumed that there was an 

 average supply of atmospheric material from the original sources, 

 external and internal. This might of course have varied, and such 

 variations must be taken into account as modifying and possibly even 

 interrupting the processes just outlined; but in discriminating the 

 effects of the latter, an average contribution from the sources of supply 

 is assumed. It is possible to build up a hypothesis of climate on 

 the variations of atmospheric supply, as will be noted later. 



