188 GROVE KARL GILBERT— DAVIS lUMMaaa iyou^a, 



sion of Gilbert's and who was especially competent to judge the quality of his astronomical 

 discussions regards them as indicating a remarkable capacity in the vizualization and the state- 

 ment of mathematical conditions and relations; for although the excursionist was little acquainted 

 with the formal machinery of higher mathematics, he was surprisingly familiar with the concepts 

 essential to their application to physical questions; he could state his problems in words with 

 remarkable facility and clarity, even though they might be difficult to express in symbols and 

 yet more difficult to work out. Nevertheless, it still seems that the necessity of tipping the 

 moon this way and that after it had essentially reached its present size, in order to explain the 

 systemless distribution of craters under the moonlet theory, is a serious embarrassment in a 

 beautifully conceived process. 



SCIENTIFIC SOCIETIES IN WASHINGTON 



Gilbert's relations to scientific societies in the decade 1881-1890 have already been out- 

 lined. The expansion of those relations a decade later is here sketched. His five-year service 

 as vice president of the Philosophical Society of Washington was succeeded by a one-year term 

 as its president, at the close of which he read the noteworthy address on the " Moon's face," 

 above noticed. He spoke before the same society in 1893 on the "Temperature of the 

 earth's nucleus," and in 1894 on "Sedimentary measurement of Cretaceous time." A dis- 

 cussion of "Gravity determinations" in 1895 will be referred to later, under the heading of 

 "Isostacy." All these subjects were eminently appropriate to the audiences before which they 

 were presented, as they were concerned with the broadest aspects of geologic philosophy. The 

 sedimentary measurement of Cretaceous time 5 was suggested by field work in Colorado, where 

 he had found sections exhibiting regular alternations of limestone and shale involving 20 or 

 more repetitions at several different levels; from this he inferred that some regular and periodic 

 change of conditions must have determined the periodic changes of deposition; and he sug- 

 gested that the change of conditions represented the cycle of the precession of the equinoxes. 

 The change was supposed to be chiefly climatic and to affect the area whence the sediments 

 found in the plains strata were derived. 



If the climate of a broad peneplain were by precession made alternately moist and dry, then during moist 

 epochs it would be densely clothed with vegetation, subterranean waters would be highly charged with organic 

 acids, so as to dissolve much lime carbonate, and mechanical degradation would be impeded by the vegetal 

 mat. During dry epochs vegetation would be sparse, water would have little power of solution, and relatively 

 rapid mechanical degradation would cause the residual clays to be transported to the ocean. 



If, then, a deposit 4 feet in thickness represented one precessional cycle of 21,000 years, 

 the total thickness of the strata concerned, 3,900 feet, would require 20,000,000 years for their 

 deposition; an estimate which, it is noted, may be half or double the truth. The value of the 

 precessional cycle as a measure of geological time was elaborated six years later in his presi- 

 dential address before the American association, mentioned below; and at about the same time 

 the use of the precessional period in the measurement of a recent time period was proposed in 

 a review" of a paper on the latest epoch of the glacial period, in which 17 terminal moraines 

 were said to occur between the farthest advance of the ice sheet at Cincinnati and its retreat 

 to the Ontario basin: 



Postulating the astromical cycle of the precession of the equinoxes as the cause of the morainic cycle, the 

 approximate time covered by the morainic history is computed ... at 315,000 years. 



As the time since the ice margin lay in the Ontario basin is measured by the age of the 

 Niagara gorge, the total time since the last ice maximum was placed at 3C0,000 or 400,000 

 years. Both of these studies represent the strong interest that their author so often mani- 

 fested in the quantitative aspect of geological problems. 



Gilbert was a member of the Biological Society of Washington, but not a frequent speaker 

 at its gatherings. On one occasion when he was present the meeting seems to have proved less 

 entertaining than he had wished; he wrote about it to a friend as follows: 



• Journ. Qeol., iii, 1895, 121-127. 

 » Journ. Qeol., vii, 1899, 621-623. 



