48 PRELIMINARY INQUIRIES. 



poraneous on both hemispheres. If the ice-sheet had covered both hemispheres, the earth's centre 

 of ji-ravity, and consequently the ocean-level, would have remained unaffected. The submergence 

 of the land is therefore another confirmation of the truth of the theory, which attributes the glacial 

 epoch to excentricity of the earth's orbit ; for, as you are aware, if the glacial epoch had been 

 due to the excentricity, the glaciation could have extended to only one hemisphere at a time. One 

 hemisphere would have been covered with snow and ice, while the other would have been enjoying 

 a perpetual spring. 



"A glacial epoch resulting from the excentricity of the earth's orbit would extend over 100,000 

 years. But owing to the precession of the equinoxes and the revolution of the apsides the glaciation 

 would be transferred from the one hemisphere to the other every 10,000 years or so. A glacial 

 epoch extending over 100,000 j^ears would therefore be broken up with five or six warm periods. 

 A warm period on the one hemisf)here would be contemporaneous with a cold period on the other. 

 Under these circumstances we ought to have elevation of the land during the warm periods, and 

 submergence during the cold. The land ought to have stood higher than at present during some 

 periods of the glacial epoch as well as lower. This, again, is in agreement with geological facts. 

 That the cold of the glacial epoch was not continuous, but was broken up by comparatively warm 

 periods, when the ice to a considerable extent at least disappeared, I think has been clearly 

 proved by Morlot, Geikic, and others, from the stratified beds of sand clay and gravel, old water- 

 courses and striated ' pavements ' which have been found in the true boulder clay." 



As regards the glacial epoch being the result of the excentricity of the earth's orbit, there 

 is much that is attractive in the idea. It would explain many puzzling facts, and others which 

 appear inconsistent with it might be explained away or reconcilod to it. For example, it may 

 be said if that is a true explanation the glacial epoch should return periodicallj^ and that this has 

 been so we have no evidence. But the heat of the earth until the glacial ej)och may have been 

 sufiicicnt to have enabled it to have endured the cold with only a slight alteration of temperature, 

 sufficient to make such a change of condition as J require for the development of new species, 

 but nothing so great as to produce an extinction of Kfe on any part of the globe : of course it would 

 be less and less felt the further back we go in the history of the earth. 



There are, however, some facts apparently opposed to it, which I do not at present see any 

 means of explaining away. The excentricity of the earth's orbit would produce its effect at regular 

 periods, always the same, and at each of these periods marks of its presence should be left — the 

 mark of its presence which I would require wovdd of course be more or less an important change 

 in the tyjjes of animal and vegetable life. These we have, but they do net recur at the right times, 

 some being separated by longer periods than others. 



Then, again, the necessary assumption that the cold did not extend to both hemispheres at the 

 same time seems inconsistent with some facts which we shall have to consider as we go along; 

 more especially the close affinity of the Arctic and Antarctic whalebone whales, whose ancestors 

 could never have passed from the one Pole to the other, unless the cold extended over the whole 

 earth to such a degree as to render the equatorial seas tolerably cold, or imless the constitution of 

 these whales was something very difierent from that of theii- descendants. 



Mr. Croll next goes on to .specidate on the thickness and weight of the ice-sheet, and the 

 extent of the effect it would produce. 



"It has been proved by Mr. Jamieson that in some parts of Scotland the ice-sheets must 

 have been at least 3000 feet thick. Agassiz thinks that in some parts of North America its 



