i6o THE POPULAR SCIENCE MONTHLY. 



trilobites entombed in the Cambrian slates. Now, just this latest 

 and present division of the geologic record, following the Ice age, 

 is the only one for which geologists find sufficient data to permit 

 direct measurements or estimates of its duration. " The glacial 

 invasion from which New England and other northern countries 

 have lately escaped," remarks Davis, " was prehistoric, and yet it 

 should not be regarded as ancient." 



In various localities we are able to measure the present rate 

 of erosion of gorges below waterfalls, and the length of the post- 

 glacial gorge divided by the rate of recession of the falls gives 

 approximately the time since the Ice age. Such measurements of 

 the gorge and Falls of St. Anthony by Prof. N. H. Winchell show 

 the length of the postglacial or recent period to have been about 

 eight thousand years ; and from the surveys of Niagara Falls Mr. 

 G. K. Gilbert believes it to have been seven thousand years, more 

 or less. From the rates of wave-cutting along the sides of Lake 

 Michigan and the consequent accumulation of sand around the 

 south end of the lake, Dr. E. Andrews estimates that the land 

 there became uncovered from its ice-sheet not more than sev- 

 enty-five hundred years ago. Prof. G. Frederick Wright obtains 

 a similar result from the rate of filling of kettle-holes among 

 the gravel knolls and ridges called kames and eskers, and like- 

 wise from the erosion of valleys by streams tributary to Lake 

 Erie ; and Prof. Benjamin K. Emerson, from the rate of deposition 

 of modified drift in the Connecticut Valley at Northampton, Mass., 

 thinks that the time since the Glacial period can not exceed ten 

 thousand years. An equally small estimate is also indicated by 

 the studies of Gilbert and Russell for the time since the last great 

 rise of the Quaternary lakes Bonneville and Lahontan, lying in 

 Utah and Nevada, within the arid Great Basin of interior drain- 

 age, which are believed to have been contemporaneous with the 

 great extension of ice-sheets upon the northern part of our con- 

 tinent. 



Prof. James Geikie maintains that the use of palaeolithic im- 

 plements had ceased, and that early man in Europe made neolithic 

 (polished) implements, before the recession of the ice-sheet from 

 Scotland, Denmark, and the Scandinavian peninsula ; and Prest- 

 wich suggests that the dawn of civilization in Egypt, China, and 

 India may have been coeval with the glaciation of northwestern 

 Europe. In Wales and Yorkshire the amount of denudation of 

 limestone rocks on which bowlders lie has been regarded by Mr. 

 D. Mackintosh as proof that a period of not more than six thou- 

 sand years has elapsed since the bowlders were left in their posi- 

 tions. The vertical extent of this denudation, averaging about 

 six inches, is nearly the same with that observed in the southwest 

 part of the province of Quebec by Sir William Logan and Dr. 



