academy of sconces ] NIAGARA AND THE GREAT LAKES 225 



glaciated mountains. But Gilbert's experience differs significantly from that of some of the 

 European geologists here alluded to in that, after extending his field of observation from a for- 

 merly glaciated to a never-glaciated area, he was open minded and clear minded enough to 

 revise his early opinions, and to give glacial erosion its due measure. As he so truly observed, 

 it is by comparing and contrasting the two types of form that the essential characteristics and 

 the origin of each can be best learned. 



MODERN VARIATIONS IN THE GREAT LAKES 



A remarkable sequel to Gilbert's studies on the shore lines of the extinct proglacial lakes 

 remains to be described. In the survey year 1896-97 he was assigned an appropriate problem: 

 " Variations in the relative height of the land in the region of the Great Lakes and the associated 

 history of Niagara River," the expectation being that the future changes in the volume of 

 Niagara might have important economic bearings. His results were briefly announced at the 

 summer meeting of the American Association at Detroit in August, 1897, and were published 

 in two forms; a popular statement in a magazine 5 that permitted his use of a partially simplified 

 spelling, and a more formal statement in an annual report of the Survey. 6 The latter opens 

 with the resurrection of an article by G. R. Stuntz, a land surveyor, " On some recent geological 

 changes in north-eastern Wisconsin," 7 which is regarded by Gilbert as broaching "for the first 

 time the idea of differential elevation in the Great Lakes region. ... It contains the only 

 observations that have ever been cited as showing recent changes of that character." Another 

 article of similar tenor which came to Gilbert's attention between the publication of the popular 

 and of the more formal statement of his work, was mentioned as " an important contribution to 

 the subject"; it was by E. L. Moseley, of Ohio, who, while Gilbert's work was in progress, 

 independently reached the conclusion that the western part of Lake Erie had recently risen. 8 

 One may see in these announcements that when it came to giving credit to his predecessors 

 Gilbert was scrupulously careful to cite their publications, yet he still remained true to his 

 principle of not claiming credit for himself. 



The method followed in seeking to determine whether earth movements in recent times 

 have affected the outlines of the Great Lakes was essentially simple, although its application 

 involved a large amount of careful observation and study. First, the present attitude of the 

 formerly level, but now slanting, surfaces recorded by the abandoned shore lines of the pro- 

 glacial Great Lakes was shown by contour lines drawn through points on the abandoned shore 

 lines having the same altitude. For these contours Gilbert proposed the name "isobases," a 

 word so new that it is not to be found in the great Oxford Dictionary. The direction of greatest 

 slant of the tilted surfaces was then immediately shown by lines crossing the isobases at right 

 angles. Next, for each of four existing lakes, pairs of shore stations were selected which lie far 

 apart and nearly on a line of greatest tilting, and at which gauges of lake levels have been main- 

 tained for the longest periods; and the gauges and records at these paired stations were per- 

 sonally examined to estimate their trustworthiness. The distances between the stations varied 

 from 78 to 186 miles; the period of their records varied from 20 to 37 years. On calculating the 

 rate of change of level for each pair of stations in feet per 100 miles per century, it was found 

 that all four pairs showed the northeastern station to be rising with respect to the southwestern 

 at the rate of 0.4 foot per 100 miles per cenhiry; or, more exactly, 0.37, 0.46, 0.39, 0.43 foot 

 for each pair. The weighted mean of the four is 0.42 foot. 



The conclusion was thereupon announced that "the harmony of the measurements and 

 their agreement with prediction from geologic data make so strong a case for the hypothesis of 

 tilting that it should be accepted as a fact, despite the doubts concerning the stability of the 

 gauges," but a candid recognition of elements of uncertainty is immediately added: "The 

 deduced rate of change . . . depends on assumptions which are convenient rather than prob- 



• Modification of the Great Lakes by earth movement. Nat. Geogr. Mag., viii, 1897, 233-247. The titles of figs. 4 and 5 are interchanged. 

 Words simplified, such as ceast, encroacht, gage, increast, markt, reacht, sketcht; but flowed, formed, planned, opposite, determine, and relative 

 are not simplified. 



• Recent earth movement in the Great Lakes region. 18th Ann. Rept. V. S. Oeol. Survey, 1S98, pt. 2, 597-647. 

 7 Proc. Amer. Assoc. Adv. Sci., rviii, 1870, 206-207. 



• Lake Erie enlarging. . . . Lakeside Magazine, Lakeside, Ohio, i, 1898, 14-17. 



