226 



[part l 



Date of last Glacial Epoch and its learing on the Measurement of 

 Geological Time. — Directly we go back from this stable period 

 we come upon changes both in the forms and in the distribution 

 of species ; and when we pass beyond the last glacial epoch into 

 the Pliocene period we find ourselves in a comparatively new 

 world, surrounded by a considerable number of species altogether 

 different from any which now exist, together with many others 

 which, though still hving, now inhabit distant regions. ■ It seems 

 not improbable that what is termed the Pliocene period, was really 

 the coming on of the glacial epoch, and this is the opinion of 

 Professor Jules Marcou.^ According to our views, a considerable 

 amount of geographical change must have occurred at the change 

 from the Miocene to the Pliocene, favouring the refrigeration of 

 the northern hemisphere, and leading, in the way already pointed 

 out, to the glacial epoch whenever a high degree of excentricity 

 prevailed. As many reasons combine to make us fix the height 

 of the glacial epoch at the period of high excentricity which 

 occurred 200,000 years back, and as the Pliocene period was 

 probably not of long duration, we must suppose the next great 

 phase of very high excentricity (850,000 years ago) to fall within 

 the Miocene epoch. Dr. Croll believes that this must have 

 produced a glacial period, but we have shown strong reasons 

 for believing that, in concurrence with favourable geographical 

 conditions, it led to uninterrupted warm climates in the tem- 

 perate and northern zones. This, however, did not prevent the 

 occurrence of local glaciation wherever other conditions led to 

 its initiation, and the most powerful of such conditions is a great 

 extent of high land. Now we know that tlie Alps acquired 

 a considerable part of their elevation during the latter part of 

 the Miocene period, since Miocene rocks occur at an elevation of 

 over 6,000 feet, while Eocene beds occur £it nearly 10,000 feet. 

 But since that time there has been a vast amount of denudation, 

 so that these rocks may have been first raised much higher 

 than we now find them, and thus a considerable portion of the 

 Alps may have been once more elevated than now. This would 

 certainly lead to an enormous accumulation of snow, which 



^ Explication d'nne seconde Edition de la Carte Geologique de la Terre 

 (1875), p. G4, 



