86 PRESIDENTS ADDRESS—SECTION €. 
ice having caused depression, and its removal the subsequent 
elevation of the land, and in this he was followed by Professor 
Shaler, in 1874. M. Adhemar and Dr. Croll had previously 
supposed that the mass of ice had attracted the water by gravi- 
tation, but it has been clearly shown that the phenomena are far 
too complicated to be explained by so simple a supposition, and, 
indeed, do not accord with it. But neither is Mr. Jamieson’s 
explanation in accord with the phenomena, for the subsidence did 
not begin until the maximum development of the ice had passed 
away, and the subsequent elevation was continued in spite of the 
second phase of the glaciation, so that the supposed effect followed 
long behind the supposed cause. In 1872 I suggested that these 
movements might have been due to the slow sinking and subse- 
quent rise of the isogeotherms, caused by the formation and 
removal of the ice. Perhaps all three causes may have acted 
together, but certainly the weight of the ice was not the sole 
cause of submersion. 
There is not much evidence in favour of elevation by unloading. 
Captain Dutton says that those regions which have suffered the 
greatest amount of denudation have been elevated most ; but this 
might equally well be put the opposite way, viz. that those 
regions which have been elevated the most have suffered the 
greatest amount of denudation. Certainly, we cannot suppose 
that mountains ever attained the height which they would now 
have if the denuded portions were restored, so that no doubt 
elevation has gone on with denudation; but the elevation must 
have been more rapid than the denudation, or else there would be 
no mountains at all; and elevation must have commenced before 
any denudation took place; consequently, denudation cannot be 
the only cause of elevation. The best case yet made out for 
elevation by unloading is Mr. Gilbert’s account of Lake Bonne- 
ville. This old Pleistocene lake was 200 by 150 miles in extent, 
but has since dried up to the comparatively small dimensions of 
the Salt Lake of Utah. The old lake margins are not Jevel now, 
but arch up over the middle of the old lake, the crown of the 
dome being some 200 feet higher than the base. Mr. Gilbert 
says that the uprising of the old lake bottom was probably caused by 
the drying up of the lake, and the unloading of a thousand feet of 
water. If this is not the cause, the dome must be part of other 
undulations which have not yet been noticed, although looked for. 
On the other hand, elevation without unloading has taken 
place during the Cainozoic era in many parts of the Atlantic 
and Pacific Oceans, as well as along the northern coast of Siberia. 
Indeed, if elevation was caused only by unloading no land would 
be elevated more than a few feet above the sea—probably there 
would never have been land at all. Depression also often 
accompanies denudation. If it were not so, no land could sink 
under the sea, and yet, undoubtedly, this has occurred many times. 
