﻿Geology and Natural History. 323 



jection marked on the Lake Survey map has been converted into 

 a strong recession, the distance of retreat here being from 200 

 to 21 5 feet in eleven years. 



Mr. G. K. Gilbert's study of the place of Niagara in geologic 

 history was in one sense a continuation of his account of the tilted 

 shore lines of old Ontario, presented at Ann Arbor a year ago. A 

 shore line is found south of Buffalo, about two hundred feet above 

 Lake Erie, and has been traced eastward as far as Alden, on a 

 branch of the Erie Railway ; the water from this high level prob- 

 ably draining into the Ohio by some outlet not yet well denned, 

 until the retreat of the ice in eastern New York allowed it to 

 escape by the Mohawk. The change from the upper to the lower 

 level was sudden, as no shore lines are known at intermediate 

 altitudes ; and this change allowed Erie to drain into Ontario by 

 the Niagara River. The conditions that might cause a variation 

 from the present rate of retreat of the falls were then discussed, 

 reaching the conclusion that the present rate is a minimum, and 

 consequently that the surprisingly low maximum of 7,000 years 

 is all that can be allowed for the excavation of the gorge. This 

 paper was illustrated by maps and sections that placed its mean- 

 ing immediately before the audience. 



The subject was further discussed by Messrs. Claypole, Davis, 

 Comstock and Holley. The first speaker compared Buffalo and 

 Chicago, illustrating a " geological might-have-been " in the con- 

 trast between their relation to the outlet of the Great Lakes, 

 which was determined by slight differences of altitude. The 

 second alluded to the contrast in form between the relatively 

 wide-open and mature old valley, filled with drift from the whirl- 

 pool to St. David's, and the narrow, steep-sided immature gorge 

 of the Niagara. Mr. Comstock discussed the influence of joints 

 in the limestone on the direction of the gorge, and Mr. Holley 

 referred to circumstances in the thickness and altitude of the 

 rocks which he thought made seven thousand years too short a 

 time for the recession of the falls. 



Topography of Chesapeake Bay. — A paper full of significant 

 detail was presented by W. H. McGee on the geography and 

 topography of the head of Chesapeake bay, in which the form of 

 the water-courses was used as a guide to the history of the re- 

 gion. The " fall-line," separating the rougher piedmont region 

 from the smoother coastal plain, was shown to divide also systems 

 of streams of strongly different characteristics ; on the first the 

 streams follow valleys well elevated above tide level, and reach 

 the bay over falls; on the second, the streams, many of which 

 run obliquely westward toward the bay, follow partly submerged 

 valleys as they enter it : the geological explanation of these facts 

 was unhappily crowded out for lack of time. 



Holyoke trap range. — Almost the only approach to petrograph- 

 ical investigation appeared in the paper by B. K. Emerson of 

 Amherst, on the Holyoke trap range, whose structure and com- 

 position were worked out with much success. The main trap 



