482 p. A. ØYEN [1914 



dannelsens længde ved Niagara at være 55440 aar; i 1841 antok 

 Lyell en varighet af 35000 aar, og atter andre har anlat Uke 

 ned til 3500 aar, eller mere almindelig 7000 — 9000 aar, enkeltvis 

 12000 aar. I 1905 — 6 antok Spenger en varighet af 39000 aar 

 »as the age of the falls (J. W. W. Spenger; The Falls of 

 Niagara, Ottawa 1907, pag. 370). »Years ago Dr. Gilbert 

 presented data, in mathematical form, demonstrå ting that the 

 Falls were no more than 7,000 years old, with several modifi- 

 cations tending to lower even this estimate« (cfr. Science, N. S. 

 Vol. 28, Nr. 726, pag. 754—759, 19H08). Det er dog Spenger, 

 som har fortjenesten af at ha brakt » Niagara out of the realm of 

 speculation* (Spenger: The Falls of Niagara, 1907, pag. 370). 

 Spenger sier selv: »I have no theory as lo the length of time 

 to defend, excepl that which is suggested by the changing 

 physical conditions, as measured bv the falls < (,cfr. Science, 

 N. S., Vol. 28. Nr. 726, pag. 754—759). Derfor linder man 

 ogsaa, at »In America the best data regarding the length of 

 post-glacial time comes from the gorge and falls of Niagara 



— 20,000 to 30,000 years as the age of Niagara < (Zeit- 



schrift fiir Gletscherkunde, B. IX, 1915, pag. 157—158). Og her 

 tinder man da ogsaa, at med korrektion af ældre maalinger 

 »finally Spencer's revision (in 1907, in which soundings and 

 borings have been most important) places their age al 39,000 

 i 4,000 years, more likely to be in excess of than below the 

 principal figure. The age of the Falls does not give us the date 

 of the close of the lee Age. From the region to the south of 

 Niagara the glaciers had withdrawn some thousands of years 

 before the birth of the Falls « (Spenger: Outline of the Evolu 

 tion of the Falls of Niagara, 1913, pag. 5). Litt senere tinder 

 vi, at CoLEMAN 19|14 tråk sammenlikning med interglacial tid 



og uttalte: »equal to post-Glacial time, say 25,000 years 



The whole of the inter Glacial interval must have been 75,000 

 or 100,000 years in length. Even if the much too short estimate 

 of post-Glacial time given by Professor Wright — 10,000 years 

 — is employed in computing the length of the inter-Glacial 

 period, it amounts to 34,000 years« (Bull. Geol. Soc. America, 

 Vol. 26, June 1915, pag. 253). Men for denne kortere tidsangi- 

 velse uttaler ogsaa f. eks. Upham sig, idet han sætter > Glacial 



and Postglacial respectively 40000—10000 



years, and 5000 — 10000 years<< (The American Geologist, Vol. 19, 

 1897, pag. 416), og senere ytrer Upham sig herom paa følgende 

 maate: »the Postglacial period, since the recession of the ice 

 sheets from the northern United States and Canada and from 

 northwestern Europe, measures about 10,000 to 5,000 years, 

 being approximately alike on opposite sides of the Atlantic* 



