academy of sciences] FIELD WORK IN COLORADO 207 



appear to have become a regular article of food. However, the single trial was a practical 

 example of what Gilbert would, in connection with accepted scientific opinions, have called 

 "going behind the postulates" ; it illustrates a readiness to control behavior by logic rather than 

 by habit; but this readiness was so well tempered on Gilbert's part that it did not make him 

 the fatiguing companion that most logic-controlled men become. All who have been in camp 

 with him testify to his sympathetic enjoyment of nature and to his unusually entertaining 

 manner of talk. He encouraged his assistants on the Colorado plains to think for themselves 

 and to form working hypotheses by asking their opinion on the meaning of new landscapes; 

 and he increased their appreciation of outdoor life by pointing out the double stars mentioned 

 in the letter quoted above, as well as the elliptical outline of the morning sun as it rose over the 

 even horizon of the plains, and the evening sky-shadow of the mountains over the shadow of the 

 earth in the east after sunset. It was a liberalizing education to work in the field with him. 

 Trouble was again found in using the base map, and one of the assistants, who had been a member 

 of the topographic branch several years before, gave "nine days to topographic sketching to 

 secure a base for the mapping of boundaries and faults in places where the engraved map proved 

 incomprehensible." 



PUBLISHED RECORDS OF WORK IN COLORADO 



The chief published products of these three seasons of field work are a report on "The 

 underground water of the Arkansas Valley in eastern Colorado," ' the Pueblo geologic folio, 

 and several short essays. The report is for the most part a straightforward geological account 

 of the successive strata in the district treated, but it contains an important generalization which 

 is outlined in the next paragraph. A small but characteristic illustration of Gilbert's clarity 

 of statement is : " The ocean is the ' base-level of erosion ' " (575), which is significant because of 

 its definiteness in contrast to the vagueness of Powell's original use of the term. Another state- 

 ment gives an account of the production of flood plains by torrential water courses: 



On the outside of each curve of its course the torrent digs into the bank, so that the river encroaches on the 

 land; but the channel does not grow broader, because in the quieter water on the inside of the curve, some of 

 the sand settles from the water and new land is thus built up. The whole flood plain of the river has been formed 

 in this way. (578). 



This simple explanatory statement is here quoted, not because of its profundity or origi- 

 nality, but because Gilbert thought such an explanatory statement was worthy of formal official 

 publication in 1896. 



Much the most important product of Gilbert's work in Colorado was the recognition of a 

 largely fluviatile origin for the fresh-water Tertiary strata of the plains, which, like similar 

 strata in basins among the mountains farther west, had been universally treated as of lacustrine 

 origin by all other geologists. A lacustrine origin had indeed been announced for the Ter- 

 tiaries of the plains in an essay in an annual report of the survey published no earlier than a 

 year before the one which contained Gilbert's discussion; but he did not treat the problem 

 historically and no reference to previous interpretations was made. The new view was briefly 

 and confidently announced, and its inferred processes were asserted as facts. The strata in 

 question were described as lying unconformably upon Cretaceous beds, the long-continued 

 erosion of which had made the region of the Great Plains "more nearly level than it is now." 

 The change from erosion of the Cretaceous to deposition of the Tertiary strata was — 



brought about by some modification of conditions which is not]yet clearly understood. Perhaps the plains region 

 was depressed at the west, and the slopes thus rendered so gentle that the streams could no longer carry off the 

 detritus which came from the mountains, and it was deposited on the way. . . . Whatever the cause, the 

 streams . . . filled their channels so that their beds lay higher than the neighboring country . . . and they thus 

 came to flow in succession over all parts of the plains and to distribute their deposits widely, so that the whole plain 

 in the district here described was covered by sands and gravels brought from the canyons and valleys of the 

 Rocky mountains. . . . The chief material is coarse sand ... in irregular beds with much oblique lamination. 



This was the beginning of an altogether new conception of the physiography of the Rocky 

 Mountain region during Tertiary time. 



i 17th Ann. Rep. U. S. Geol. Survey, 1896, Pt. II, 557-601. 



