Niagara Falls 



1898 beaches have been discovered by Prof. Fairchild. Another 

 Spencer diagnosis of the glacial lake is the occurrence of gravel floors 



over low divides, which are regarded as the outlets of them, and 

 upon this feature alone many such lakes have been named. But 

 the advocates of these glacial outlets have not explained how the 

 terraces (at hundreds of feet above the drainage) upon the 

 southern side of them are indistinguishable in character from 

 those upon the northern side. 1 If these supposed outlets be 

 evidence per se of glacial dams then the most perfect which the 

 writer has ever seen may be found within 16° of the equator, 

 at an altitude of less than 800 feet, suggesting that the Mexican 

 gulf had a glacial dam, discharging into the Pacific ocean across 

 the Isthmus of Tehuantepec — a suggestion which no one would 

 seriously consider. The writer has also presented the hydrostatic 

 objections 2 to the impossible long continuance of some of the 

 supposed dams, the location of which demands their drainage 

 across ice itself, which would soon be penetrated by the warmer 

 waters so as to reduce their level. By straightening out the 

 deformation recorded in the deserted shore-lines, some of the 

 beaches are shown to have undoubtedly been formed at sea 

 level. While recent surveys report the discovery of additional 

 glacial lakes, or the splitting up of those first described under 

 new names, the survey of the high level terraces in the mountain 

 regions has suggested to the writter counterbalancing evidence of 

 the occurrence of glacial dams, but this is a study which has 

 been postponed, partly on account of the prejudice against post- 

 glacial subsidence and partly on account of the writer's absorption 

 in other questions of physical changes. Whatever may be the 

 ultimate fate of the theory of glacial dams, the opposing 



1 Channels over divides, etc., Spencer, Bull. Geol. Soc. Am., vol. Ill, 

 1891, p. 491. 



2 Post-Pliocene continental subsidence versuc ice dams. Spencer. 

 Ibid., II. pp. 465-476, 1890. 



3 The Iroquois beach, etc. 



618 



