208 GROVE KARL GILBERT— DAVIS [Meuoirs [ v'o a l t xxi; 



Gilbert himself had in earlier years accepted without particular inquiry the orthodox 

 lacustrine interpretation of certain fresh-water Tertiaries in the high plateaus of Utah; but 

 his subsequent experience with the Quaternary clays and marls of Lake Bonneville gave him a 

 better understanding of the composition and structure by which lacustrine deposits, laid down 

 in quiet and comparatively deep water, must be characterized. There is, however, nowhere 

 any indication that he, while working in Bonneville, harked back to the Tertiaries of Utah 

 plateaus and saw that, in view of their frequently varied texture and irregular structure, an 

 altogether lacustrine origin should not be attributed to them. It was not until later, when he 

 came to know the obliquely laminated, sandy strata of the plains, that their contrast with the 

 regular stratification of the fine-textured Bonneville sediments appears to have compelled an 

 interpretation chiefly by fluviatile processes; and even then nothing was said regarding a similar 

 origin for any of the Tertiary formations in the intermont basins. A year later Haworth, after 

 quoting extracts from Gilbert's Colorado report, extended the fluviatile explanation eastward 

 into Kansas; and in 1900 Johnson brought out his comprehensive essay on the high plains, 2 

 in which the brief explanatory statements made by Gdbert about the Tertiaries of eastern 

 Colorado were expanded in a truly philosophical manner and extended over a much larger area. 



The shorter articles that Gilbert based on his studies of the Plains concern the measurement 

 of Cretaceous time, elsewhere referred to, some small laccoliths to which reference has already 

 been made, and, jointly with one of his assistants, an account of "Tepee buttes," 3 or small 

 residual eminences like Indian wigwams or "tepees" in form, which owe their survival over 

 widespread surfaces of degradation to the presence of colonies of fossil shells. Gilbert gave 

 much attention to fire clays during this period, and many samples acquired by correspondence 

 were analysed for him; but no report of the results gained was published. The average com- 

 position of sedimentary rocks was another subject that he took up with the assistance of the 

 chemists of the survey during these years; it was as if his active mind did not find sufficient 

 occupation in examining the simple structure of the horizontal strata of the plains, and he was 

 therefore driven to consider certain large problems in connection with their origin. Much time 

 must have been given to measurements and calculations, but the statement of the results reached 

 was published only in abstract; its chief conclusion was that "assuming the sedimentary rocks 

 to have been supplied by erosion from the crystallines . . . somewhat more than a mile in 

 thickness of crystalline rocks upon areas equal to all the present land of the globe must have 

 been worked over to give our sedimentary rocks." 4 All these subordinate problems are good 

 enough in their way, but they were rather random and discontinuous efforts; and that they 

 should have taken Gilbert's attention from larger problems was as inappropriate as that he 

 should have had, when he was chief geologist, to advise other members of the survey as to the 

 manner of keeping their accounts and as to the choice between buying a bicycle and hiring a 

 buggy for transportation. 



THE PUEBLO GEOLOGIC FOLIO 



Only one geologic folio, that of the Pueblo quadrangle, was published under Gilbert's author- 

 ship. The Apishapa quadrangle adjoining on the southeast was surveyed under his direction 

 in 1894, but the folio was not published until 1912, when it was edited by one of Gilbert's assist- 

 ants after revision in the field. Besides the standard series of maps, the Pueblo folio contains a 

 stereogram made under Gilbert's direction, representing the warped and faulted surface of a 

 standard stratum, uncovered where still buried and restored where already eroded, and also a 

 special map indicating by underground contours the depth of the uppermost water-bearing 

 strata. It will be remembered that, as to stereograms, Gilbert had nearly 20 years before 

 prepared two excellent examples for the illustration of his Henry Mountains report, and that 

 these plates had inspired the preparation of similar illustrations by de la Noe" and de Margerie 

 for their "Formes du terrain"; it may be added that this Pueblo stereogram is probably the 

 best example of its kind that has been produced since the Henry Mountains report appeared. 



> W. D. Johnson, The High Plains and Their Utilization. 21st Ann. Report., U. S. Geol. Survey, 1901. 



• Bull. Geol. Soc. Amer., vi, 1895, 333-342. 



' The chemical equivalence of crystalline and sedimentary rocks. Amer. Geol., mi, 1894, 213-214. 



