482 p. A. OYEN 1191-4 



dannelsens længde ved Niagara al være 55440 aar; i 1841 anlok 

 Lyell en varighet af 85000 aar, og atter andre har antal like 

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

 12000 aar. I 1905 — 6 anlok Spencer en varighet af 39000 aar 

 »as the age of the falls (J. W. W. Spencer: The Falls of 

 Niagara. Ottawa 1907, pag. 370). Years ago Dr. Gilrert 

 presented data, in mathematical form, deinonstrating Ihat the 

 Falls were no more Ihan 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, 19f j08). Det er dog Spencer, 

 som har fortjenesten af at ha hrakt Niagara out of the realm of 

 speculation- (Spencer: The Falls of Niagara, 1907, pag. 370). 

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

 lo defend, excepl thai 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 finder 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 flir Gletscherkunde, B. IX, 1915, pag. 157—158). Og her 

 finder man da ogsaa, at med korreklion af ældre maalinger 

 »finally Spencers revision (in 1907, in which soundings and 

 borings have been most imporlanl) piaces their age at 39,000 

 J_ 4,000 years, more likely lo be in excess of than below the 

 principal figure. The age of the Falls does nol give us the dale 

 of the close of the Ice Age. F/om Ihe region lo the soulh of 

 Niagara the glaciers had wilhdrawn some thousands of years 

 before the birtli of the I"alls< (Spencer: Oulline of the Evolu 

 lion of the Falls of Niagara, 1913, pag. 5). Lill senere finder 

 vi, at Coleman 19|14 trak 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 lenglh. 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, il amounts lo 34,000 years« (Bull. Geol. Soc. America. 

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

 velse utlaler ogsaa f. eks. Upham sig, idet han sæller »Glacial 



and Postglacial respectively 40000— lOOOO 



years, and 5000 — 10000 years^ (The American Geologisl, Vol. 19, 

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

 maale: »the Poslglacial period, since Ihe recession of the ice 

 sheels from the norlhern United States and Canada and from 

 northweslern Furo|)e, measures aboul 10,000 lo 5,000 years, 

 being approximalely alike on opposile sides of Ihe Allantic« 



