T. F. Jamieson — Changes of Level in the Olacial Period. 347 



tlie ice, to the other that it overestimates the flexibility of the earth's 

 ci'ust. Here both causes operate to produce the same result ; but if 

 we could find a case where we could test the two under conditions 

 where the results would be difi'erent or opposite, then it might be 

 seen which would prevail. 



Such a case would seem to be found in another great lake called 

 Lake Bonneville, which formerly existed in the salt desert of Utah 

 during the Quaternary period. This lake arose from a change in the 

 climatic conditions of that region. Now it is so dry that the water 

 has evaporated from the greater part of the basin, leaving it a salt 

 desert; but formerly the precipitation of moisture was so much 

 greater that the basin was filled by a body of water as large as Lake 

 Huron, and nearly 1000 feet deep. Here, then, we have a tract of 

 ground which formerly sustained the pressure of a heavy load of 

 water that has now disappeared. If the surface of Lake Bonneville 

 was influenced by the attraction of surrounding masses, its waters 

 should have stood highest at their margin, where they adjoined the 

 mountain ranges which encompassed them, and its shore-lines (which 

 are still remarkably distinct) ought to find their greatest altitude in 

 such places. On the other hand, if the land rose in any degree 

 when the load of water was taken ofi" it, the old beaches should now 

 be highest towards the centre of the basin where the water was 

 deepest and the load heaviest. Whereas, at the outer rim of the 

 lake, where the water was shallow, no material rise might be expected. 

 This, I think, is a fair inference in regard to both theories. Now, 

 what do we find ? 



According to Mr. G. K. Gilbert,^ who has devoted much time and 

 labour to the exploration of the locality, the answer would seem to 

 be decidedly in favour of the latter supposition. There were 

 numerous islands in this old lake, and its shore-line encircles them 

 so distinctly that its level can be ascertained not only round the 

 margin, but also in the midst of the basin. This shore-line is not 

 now level, but rises higher in some parts than in others. A map 

 of the lake was made, its shore-lines carefully traced, and the 

 altitudes marked upon the map. The final result of the whole 

 survey went to show that the highest parts of the beach lie within 

 the area of the main body of the lake, while the lowest ones are at 

 the extreme north and south ends, and the general expression of the 

 deformation of the surface indicated a low broad dome of elevation 

 to have taken place, having its crest situated over the centre of the 

 main body of the lake. The difference of level between the margin 

 of the main body of the lake and its centre is not less than 100 feet, 

 but including observations on outlying bays it amounts to 168 feet. 

 Gilbert, therefore, believes that the bottom of the lake has risen 

 since the water disappeared, and he seems to think it probable that 

 the cause of this is to be found in the removal of the load of water 

 that formerly lay upon it. He discusses two other theories, but finds 

 them both inadequate to account for the facts. 



1 G-. K. Gilbert, " On the Inculcation of Scientific Method by Example," Amer. 

 Journ. of Science, vol. 31, April, 1886. 



