62 The American Geologist. Juiy,i894 
Al'icr all, it is mil surprising that Prof. Tarr should misunderstand 
me. for it appeal's from his own statement that, al the time Dr. 1). F. 
Lincoln's paper, "Glaciationof the Finger-Lake Region of New York,"* 
was published, 7it was so strongly convinced that tin valleys were river val- 
leys, unmodifiedby ice, that his [Lincoln's] paper produced no impression. 
It was only when the clearness of the evidence impressed itself so forci- 
bly on his mind that he could be but convinced thai he again looked at 
his [Lincoln's] paper, and found thai he had the same kind of proof. 
Therefore, although independently worked out, the facts in this article 
[Tarr's "Lake Cayuga a Rock Basin"] are merely -confirmations of Dr. 
Lincoln's study and deductions.! Frederic W. Simonds. 
University of Texas, Austin, April 12, 1894- 
The Niagara gorge as a Measure op the Postglacial period. 
In Nature for May 17th (vol. 50, p. 53) Mr. G. K. Gilbert expresses his 
doubt that the past rate of erosion of the gorge below the receding falls 
of Niagara-can be so compared with the presenl rate as to afford any 
approximate measure of the time which has elapsed since the Ice age. 
His first study of this question (Proc. A. A. A. S., vol. xxxv, for 1886, 
pp. 222, "22:!) gave about 7.000 years for the erosion of the Niagara gorge, 
if it had proceeded with anaverage rate like the present. As this erosion 
began at the time of retreat of the ice-sheet from that, area and has 
been in progress 1 during all the subsequent time, it has been regarded as 
a geologic chronometer, like the similar erosion of the gorge below the 
falls of St. Anthony, from which Prof. N. H. Winched had previously 
computed the length of the Postglacial period in Minnesota to be about 
7,800 years. 
The largest element of uncertainty (as hitherto supposed) in the esti- 
mate drawn from the rate of recession of Niagara falls is shown by Mr. 
Gilbert's later and more full discussion (Sixth Annual Report of the 
Commissioners of the State Reservation at Niagara, for the year 1889, 
pp. 61-84, with eight plates, also in the Smithsonian Annual Report for 
1890) to consist in the probability or possibility that for some consider- 
able time, next following the melting away of the ice upon the area 
crossed by the Niagara river, the outlet of lakes Superior, Michigan and 
Huron, may have passed to the St. Lawrence by a more northern course, 
flowing across the present watershed east of lake Nipissing to the Mal- 
tawa and Ottawa rivers. Since Mr. Gilbert's writing his recent com- 
munication to Nature, however, much new lighl on the Quaternary 
history of the greal lakes tributary to the St. Lawrence river has been 
contributed in three papers by Mr. F. B. Taylor, published in the Bul- 
letin of the Geological Society of America (vol. v. pp. 620-626, April 30, 
1894)4 and in tin' .May and June numbers of the AMERICAN GEOLOGIST. 
Supplementing the earlier observations and studies of Whittlesey, New- 
berry, Gilbert, spencer. Lawson, Chamberlin, Leverett, Claypole, 
*Am. Jour, of Science, III, vol. xliv, pp. 290-301, Oct. 1892. A later paper on this 
subject by Dr. Lincoln is in vol. xlvii, pp. 105-113. Feb., 1894. 
tBulletin G. 8. A., vol. v, p. 348, foot-note. 
^Abstract in the March Am. Geologist, p. 220. 
