238 THE GLACIAL LAKE AGASSIZ. 



Lam , Spluerium sulcatuni Lam., and (.Tiji-aidus parvus Sav. These species 

 from both hicahties were kindly determined l)v Prof. E. Ellsworth Call, 

 who states that Unio luteolus is one of the most widely distributed repre- 

 sentatives of the genus, its range being from Lake Winnipeg to Texas, east 

 to New York, and west to Montana. It is generally abundant in jMinne- 

 sota. Both these species of Spliceriuni are reported bv Dr. Dawson from 

 the Lake of the Woods and Pembina River; and the fii-st is the mo.st com- 

 mon species of its genus in Minnesota, while its rang-e northward extends 

 at.least to Great Playgreen Lake and York Factory, where it has been col- 

 lected by Dr. Bell. The Camjibell beach was fonned in the later part of 

 the time of the lake's southward outflow; and the Gladstone beach belongs 

 to the middle portion of the time of its outflow toward the northeast, its 

 south eud being then about 90 miles south of the international boundary. 



MEASUREMENTS OF TIME SIIfCE THE GLACIAL PERIOD. 



If the question be asked, How many thousand years ago did the reces- 

 sion of the ice-sheet take jjlace, causing Lake Agassiz to fill the Red Rivet* 

 Valley and the basin of Lake Winnipeg? a reply is furnished by the com- 

 putations of Prof. N. H. Winchell,^ that approximately eight thousand years 

 have elapsed during the erosion of the postglacial gorge of the Mississippi 

 from Fort Suelliug to the Falls of St. Anthony; of Dr. Edmund Andrews,^ 

 that the erosion of the shores of Lake Michigan, and the resulting accumu- 

 lation of dune sand di-ifted to the southeni end of that lake, can not have 

 occupied more than seven thousand five hundred years; of Prof. G. Fred- 

 erick Wright,^ that streams tributary to Lake Erie have taken a similar 

 length of time to cut their valleys and the gorges below their waterfalls; 

 of Mr. G. K. Gilbert,^ that the gorge below Niagara Falls has required only 

 seven thousand yea,rs or less; and of Prof. B. K. Emerson,^ on the rate of 



' Geology of Minnesota, Fifth Annual Report, for 1876; and Final Report, Vol. II, pp. 313-341. 

 Quart. Jour. Geol. Soc., Vol. XXXIV, 1878. pp. 886-801. 



-Transactions of the Chicago Academy of Sciences, Vol. II. James C. Southall's Epnch of the 

 Mammoth and the Apparition of Man upon the Earth, 1878, Chapters XXJI and XXIIl. 



••'Am. Jour. Sci. (3), Vol. XXI, pp. 120-123, Feb., 1881; The Ice Age in North Americ.i, 1889, Chap- 

 ter XX. 



■* Proc. A. A. A. S., Vol. XXXV, for 1886, p. 222. " The History of the Niagara River," Sixth An. Rep. 

 of Commissioners of the State Reservation at Niagara, for 1889, pp. 61-84. 



*• Am. Jour. Sci. (3). Vol. XXXIV, pp. 404, 405. Nov., 1887. 



