ox THE COAST OF MAINE. 335 



the land areas. Moreover, we may hope to get, through the study of these I'ehcs of the 

 past glacial action, a means of determining the identity of formations at jioints remote from 

 each other. If there should, for example, be evidence that at various points on the earth's 

 surface rocks of supposed triassic age, as determined by their fossil contents, are associated 

 with distinct evidences of glacial action, we may feel that the identification of age is more 

 exact by far than it can be made on the evidence derived from fossils alone. That similar 

 organic forms can continue to exist in one region long after they have been extinguished in 

 another, is a fact so well I'ecognized as to need no proof; but a glacial period, we ai'e 

 entitled to believe, must have been simultaneous, at least throughout one hemisphere, prob- 

 ably over both, and may therefore give with certain lessened risks of error, a means of 

 determining identity of time in regions remote from each other. 



ox THE CAUSE OF DEPRESSION IX THE GLACIAL PERIOD. 



That there is a necessary connection between the accumulation of great masses of ice, 

 such as the glacial periods brought upon the surfixce of the cii'cumpolar regions, and the 

 depression of those regions, is a point most easily demonstrated. Over the whole of the 

 shore of the North Atlantic the evidence is complete ; at the southernmost point where the 

 glacial action has been observed, we find slight evidences of subsidence. This increases 

 steadily, though irregularly, as we go northward, until we come to the highest latitudes 

 where civilized man has penetrated, where we have evidences of subsidence amounting to 

 two thousand feet or more. The only hypothesis as yet advanced to account for this irreg- 

 ular movement, is that of which Adhemar was the originator. 



The accumulation of a great weight of ice at either pole in succession, would result m 

 the disj)lacenient of the centre of gravity of the earth, which would be draAvn the nearer 

 to the pole where the accumulation took place. The result of tliis change upon the alti- 

 tude of the sea would be veiy great. The water being free to obey the changed conditions 

 would rearrange itself with reference to the new centre of attraction. The result would 

 be the dee^^ening of the sea by the amount of the displacement of the centre of gravity 

 about the pole where the ice accumulated. That such a result is in good degree a neces- 

 sary consequence of the accumulation of masses of ice al)out either one or the other pole, 

 is easil}" believed. But there is every reason to suspect that the conditions Avere not ful- 

 filled during the last glacial period. The observations of Darwin, and more recently, and 

 more especially, of Agassiz, have shown that the southern extremity of South America 

 bears marks of as recent and extensive glaciation as the Northern Hemisphere. Until it is 

 proven to the contrary by evidence which has not been seen, we must suppose that the 

 glaciation in the two hemispheres was simultaneous. If this be the case, the idea of a dis- 

 placement of the centre of gravity is no longer so available as an explanation ; for although 

 the tendency to accumulate the seas about the poles would exist in a very diminished way 

 under this condition, its amount would not be sufficient to account for the phenomena of 

 depression. Moreover, any result of this kind would probably be more than compensated 

 by the substrata of material from the sea, and the consequent reduction of its depth. If 

 we suppose both hemispheres laden with ice down to the parallel of 45% then the thickness 

 to be on the average one mile, we shall have subtracted from the ocean water enough to 

 lower its depth by over half a mile of altitude. Something of this would be compensated 



