148 POPULAR SCIENCE MONTHLY. 



The iudetermiuate question which remained was, At what rate 

 did this postglacial elevation of land which has brought it up to 

 its present level proceed? Dr. Gilbert, Professor Spencer, and Mr. 

 Tajlor have brought forth a variety of facts which, according to 

 their interpretation, show that this rate of elevation was so slow that 

 from twenty thousand to thirty thousand years was required to restore 

 to the iSfiagara Kiver its present volume of water. Their arguments 

 are based upon the varying width and depth of the Niagara gorge, 

 proving, as they think, the presence of a smaller amount of water 

 during the erosion of some portions. Dr. Gilbert has also brought 

 forward some facts concerning the extent of supposed erosion pro- 

 duced by the diverted waters of l^iagara when passing over an 

 intermediate outlet between Lake Simcoe and Lake Nipissing. But 

 the difficulty of obtaining any safe basis for calculation upon these 

 speculative considerations has increased the desire to find a means of 

 calculation which should be independent of the indeterminate prob- 

 lems involved. That I think I have found, and so have made a 

 beginning in obtaining desired results. The neiu evidence lies in 

 the extent of the enlargement of the mouth of the Niagara gorge at 

 Lewiston since the recession of the falls hegan. 



It is evident that the oldest part of the Niagara gorge is at its 

 mouth, at Lewiston, where the escarpment suddenly breaks down to 

 the level of Lake Ontario. The walls of the gorge rise here to a 

 height of three hundred and forty feet above the level of the river. 

 It is clear that from the moment the recession of the falls began at 

 Lewiston the walls of the gorge on either side have been subject to 

 the action of constant disintegrating agencies, tending to enlarge 

 the mouth and make it V-shaped. Wliat I did last summer was to 

 measure the exact amount of this enlargement, and to obtain an ap- 

 proximate estimate of the rate at which it is going on.* As this en- 

 largement proceeds wholly through the action of atmospheric agen- 

 cies, the conditions are constant, and it is hoped that sufficiently 

 definite results have been obtained to set some limits to the specula- 

 tions which have been made upon more indefinite grounds. 



The face on the east side of the gorge presents a series of alternate 

 layers of hard and soft rocks, of which certain portions are very sus- 

 ceptible to the disintegrating agencies of the atmosphere. The sum- 

 mit consists of from twenty to thirty feet of compact Niagara lime- 

 stone, which is underlaid by about seventy feet of Niagara shale; 

 which in turn rests upon a compact stratum of Clinton limestone 



* For opportunity to do this work I am indebted to the interest of President S. R. 



Callaway, of the New York Central Railroad. The measurements were made by Mr. George 



S. Tibbits, engineer of the western division. The photographs were taken by Mr. C. F. 

 Dutton, of Cleveland. 



