ACADEMY F SCIENCES] SCIENTIFIC ACTIVITIES 189 



Last evening I attended a meeting of the Biological Society. Prof. Osborn of Princeton [later of the 

 American Museum of Natural History in New York City] presented a review of Marsh's recent papers on the 

 discovery of Cretaceous mammals. Merriam, president of the Society, had sent a notice of the meeting to 

 Prof. Marsh and he was there to hear himself reviewed. Osborn undertook to show that Marsh had made 

 seven species, five genera, and four families out of the teeth of one species — and more of the same sort. Marsh, 

 in reply said that when he wrote he had about 100 specimens, but that the work of collection had not stopped 

 and now he had more than 1,000, including some jaws with the teeth in place. This later material was not 

 fully studied but he was able to say that Osborn was partly right and partly wrong — which I thought, con- 

 sidering Marsh's peculiarities, a remarkably good response. So all our fun was quiet, and the proceedings of 

 the Society on the Stanislaus were not repeated. 



No explanatory notes will be needed by any reader of tbis memoir as to tbe biologists above 

 named; but as some of tbe younger readers may be puzzled by tbe mention of tbe "Society 

 on the Stanislaus," it may be well to state tbat its publications are not listed in tbe Royal 

 Society's catalogue. 



The allusion here made to Marsh may introduce a reference to the second of the few 

 papers that Gilbert wrote on what seemed from its title, "Age of the Potomac formation," 7 to- 

 have to do with historical geology or with paleontology, but which proves on examination to 

 be concerned chiefly with those aspects of physical geology that Gilbert habitually considered 

 and, indeed, rather with the philosophy of that division of geology than with its facts. It 

 appears that Marsh had assigned the so-called Potomac formation — a nonmarine deposit at 

 the base of the Atlantic coastal plain series — to the Jurassic; and Gilbert asked for the "char- 

 acter of the evidence and the course of reasoning" on which the assignment was based. He 

 was prompted to do so because in the absence of marine invertebrate remains, Marsh had been 

 guided by vertebrate fossils, and the method by which such fossils are interpreted as time 

 markers "must differ in an important way from the method ordinarily used by students of 

 invertebrate fossils and fossil plants. As he [Marsh] has pointed out, land vertebrates are 

 peculiarily sensitive to climatic and other physical conditions, and the evolution of new forms 

 is consequently rapid. The life of a species is short and its value for purposes of correlation 

 . . . correspondingly high. . . . But," Gilbert adds, "it appears to me a priori that this 

 quality of rapid evolution is a two-edged sword," because it prevents correlation by widespread 

 identity or close similarity of forms. Moreover, in point of fact the Potomac beds in New 

 Jersey lie unconformably on Triassic strata below and pass insensibly into marine Cretaceous 

 above; and Marsh seems to have interpreted these structural relations as indicating an inter- 

 mediate time relation between Triassic and Cretaceous. 



The geologist, however, infers that the unconformity beneath the Potomac represents a time interval, 

 and consideration of the extensive dislocation and deformation of the Newark [Triassic] beds and of the enor- 

 mous degradation they suffered before the deposition of the Potomac gives the impression that the time- 

 interval was very long as compared to the time represented by the Potomac beds themselves. 



In closing, Gilbert remarked that he had "no intention to controvert Prof. Marsh's view, 

 but merely to show how desirable it is that he set forth the reasons therefor." 



A later communication was made to the Biological Society of Washington in 1899, regard- 

 ing the effects of a landslide by which the Columbia River had been dammed at the Cascades, 

 "not less than 350 years ago"; the observations on which this study was based were made on 

 the return from the Harriman Alaska expedition, and the results were presented to a biological 

 society because they had to do with the explanation of a submerged forest. 8 



The National Geographic Society had Gilbert's services on its board of managers from 

 1891 to 1900, except when he acted as vice president in 1896 and 1897. He addressed its, 

 meetings on various occasions, his subjects being "Coon Butte," in 1892; the "Great Basin,"' 

 in 1898; and the "Glaciers of Alaska," in 1899. He also gave, in January, 1894, under the 

 auspices of this society six lectures on the "Shaping of the earth's surface," apparently repeated 

 from a similar course for Washington teachers in the last months of 1892. He contributed a 



' Science, iv, 1896, 876-877. 

 • Science, xi, 1900, 99, 100. 



