112 GROVE KARL GILBERT— DAVIS tMBMOtES [ ^S 1 ^ 



It would appear from the last sentence that the possibility of the guidance of unsymmotrical 

 laccolithic intrusions by faults, which has been recently suggested as an explanation for all 

 such bodies, had by no means been overlooked by Gilbert, but that its apphcation to the typical 

 laccoliths of the Henry Mountains, which was included in the recent suggestion would not have 

 been acceptable to him. Later in the same season he wrote to his chief from Kanab, in southern 

 Utah: "I am anxious to hear all about the Nat. Acad, and the consolidation of surveys," thus 

 showing that he was apprised of the movement made by Powell toward abolishing the separate 

 departmental surveys and uniting all geological work of the Government in one organization, 

 as was done two years later. 



The results of the season's field work in 1877 are embodied in two chapters that Gilbert 

 prepared for Powell's Lands of the Arid Region (1879). One gives a methodical account of the 

 "irrigable lands of the Salt Lake drainage system"; the other discusses the water supply of the 

 same area in an analytical manner thoroughly characteristic of its author. It is to be doubted 

 whether any other discussion of this subject in more recent years, extended as it may now be by 

 longer records of rainfall and of stream flow, is so complete as Gilbert's in the way of examining 

 every item of cause, every step in argument. His discussion is based on recorded variations in 

 lake level and area for the 30 years following the time of Mormon settlement in 1847, on measure- 

 ments of certain streams, and on various general considerations, among which the human 

 agencies of farming, grazing, and tree cutting are prominent. A rise of lake surface as the result 

 of an upheaving deformation of the lake bottom is definitely excluded, because any resulting 

 tendency to increase of area would be counteracted by increased evaporation, unless a change 

 toward moister climate occurred at the same time. Human agencies are not accepted as affect- 

 ing precipitation, in spite of the widespread popular opinion to that effect; but they are thought 

 to diminish evaporation and to increase snow melting and run-off by small amounts, and thus 

 possibly to increase stream inflow into the lake more than it had been decreased by evaporation. 

 The chief cause of lake increase was provisionally held to be climatic. The increase of lake area 

 between the surveys by Stansbury at a seasonal low-water stage in 1850 and by King at a seasonal 

 high-water stage in 1869, when the lake covered 1,750 and 2,166 square miles, respectively, is 

 interpreted to represent an increase of 17 per cent from a mean stage of 1,820 square miles at the 

 earlier date to one of 2,125 square miles at the later date; and the areal increase of 17 per cent is 

 estimated to represent a 10 per cent increase of rainfall. It is then concluded that while "the 

 hypothesis which ascribes the rise of the lake to a change of climate should be regarded as 

 tenable ... it can claim no more than a provisional acceptance." A test of this hypothesis 

 to-day is made difficult if not impossible, because the great shrinkage of lake area in recent 

 years, usually ascribed to an increase in irrigation, has rendered the lake next to useless as a 

 gauge of climatic change. 



TRIANGULATION IN UTAH AND ARIZONA 



The field season of 1878, lasting from July to December, was given to a task that few 

 geologists would have felt themselves capable of executing: Nothing less than designing and 

 executing a system of triangulation as the basis for a topographic survey of the plateau province, 

 covering a part of the high plateaus in the north, the Colorado plateaus on both sides of the 

 Grand Canyon farther south, and some of the adjacent basin ranges. The field party included, 

 besides Gilbert in charge, Renshawe, Bodfish, Hillers, and a number of others. The geodetic 

 work involved the connection of two base lines previously measured, one at Gunnison near the 

 western base of the Wasatch Mountains, about 120 miles south of Salt Lake City, the other at 

 Kanab, as much farther south still and close to the southern boundary of Utah; also the 

 remeasurement of the latter base by an ingenious apparatus of Gilbert's own design, constructed 

 under his personal supervision. A letter to the "Major" written before the party was sub- 

 divided, told of "having all the new boys drilled in barometric work," running levels to connect 

 barometric stations, and setting up "aparejoes" or pack saddles and half converting a skeptical 

 member of the party to their utility; then of sending off one division of the party with wagon 

 and pack train southward to cross the Colorado below the mouth of the Grand Canyon and 



