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GEOGRAPHICAL AND GEOLOGICAL SURVEY OF THE ROCKY
DEPARTMENT OF THE INTERIOR,
ROCKY MOUNTAIN REGION,
Washington, D. O., November 25, 1877. SIR: I have the honor to submit berewith a brief abstract of the operations of the Survey under my direction.
The first part relates to the field operations for the fiscal year ending June 30, 1877. This occurs from the fact that in pirties did not arrive from the field until about the 1st of January, 1877; too late to prepare a statement for the last annual report of the Secretary.
FIELD SEASON OF 1876. As soon as the appropriation for the fiscal year of 1876–77 could be used, the surveying corps left Washington and proceeded to the rendezvous camp at Gunnison, Utah, where the field-parties were organized, under the general superintendence of Prof. A. H. Thompson, geographer of the expedition. While en route they were joined by Capt. Clarence E. Dutton, of the Ordnance Department V. S. A., who had been assigned for duty with this Surrey by the Secretary of War, and directed to make an examination of the immense fields of igneous rocks in Southeastern Utah.
The field organization as finally completed, differed somewhat from that of previons years, the geograpbic and geological work being assigned to separate parties, each practically independent in all movements though working under the same general plan and witbin the same territorial limits. It is believed that better results can be, and bave been secured by this separation of distinct branches of the survey than by the old method of attaching a geologist to a geograpbic party or a geographer to a geological party.
Five parties were organized : one under Prof. A. H. Thompson to continue the triangulation; one topographic party under Mr. Walter H. Graves, apother under Mr. Johu H. Renshawe; one geological party, under Mr. G. K. Gilbert, another under Capt. C. E. Dutton.
TRIANGULATION BY PROFESSOR THOMPSON. · The party under Professor Thompson continued the expansion of the primary triaugulation resting on the base lines measured in preceding years at Kanab and Gunnison, Utah. The area embraced in this season's work amounts to about 10,000 square miles.
TOPOGRAPHIC WORK BY MR. GRAVES. Topographic party No. 1, in charge of Mr. Graves, extended the secondary triaugulation over an area of 6,000 square miles, lying between
the Wasatch Mountains on the west and the Green and Colorado Rirers on the east. Mr. Graves also made a complete plane-table sketch of the country surveyed, which, taken in connection with bis angles for locations and perspective profile sketches, will enable hiu to construct a map of his district on a scale of 4 miles to the inch. The principal topographic characteristics of this region are long lines of anscalable cliffs, tbe escarped edges of terraced plateaus, of which the country is composed, and deep, narrow cañons, with vertical walls, both presenting well-nigh impassable barriers to travel.
The only considerable boilies of irrigable lands found are along the valleys of the Green and San Rafael Rivers. The only tiinber lauds are on the Sevier plateaus at an elevation from 8,000 to 11,500 feet.
TOPOGRAPHIC WORK BY MR. RENSHAWE. The work of topographic party No. 2, under Mr. Renshawe, was con: fined to Southwestern Utah and Southeastern Nevada, one of the most rugged and barren sections in the Great Basin. The methods of survey were the same as adopted by party No, 1, except that perspective pro. file sketches were made by the aid of the orograph, a newly designed instrument that proinises to be of great use iu topographic surveying. The work of Mr. Reushawe and his assistant, Mr. O. D. Wheeler, was extended over about 4,000 square miles. In all this area no considerable bodies of irrigable lands are found; probably not one-half of one per cent. possessing any value except for pasturage.
TOPOGRAPHIC WORK BY MR. GILBERT. A topographic survey of the Henry Mountains was made in 1875, and a map constructed on a scale of 4 miles to tbe inch; but this being thought too small a scale to admit of correct representation of the details of the geology, Mr. Gilbert in addition to his geological work made a more detailed survey of the topography, carrying a complete system of secondary triangulation and a connected plane table sketch over more than 1,000 square miles. The data collected are sufficient to make a topographic map of the Henry Mountains on a scale of 2 miles to the inch, or 63007 DIVISION OF THE AREA INTO DISTRICTS FOR MAPPING PURPOSES.
The Rocky Mountain Region of the United States (not including Alaska), or that portion west of the meridian of 990 30', was by a former Secretary of the Interior divided into districts for surveying and maj). ping purposes and these districts numbered. The area of each district is 24 degrees in longitude and 14 degrees in latitude. The region of country surveyed by the parties under my direction is embraced iu districts numbered 75, 85, 86, 95, 96, 104, and 105 (see accompanying map), the first five lying directly west of the regiou in which Dr. Hayden was engaged, while districts 104 and 105 lie immediately south of the other districts in which my own parties have been at work. During the ear. lier part of the work, before these districts were established by the Department, the operations of the Survey extended in an oblique direction from nortbeast to southwest along the general course of the Green and Colorado Rivers, through the districts above designated, but the work was in such condition that no one district was complete. During the season my parties were engaged in extending the survey over the unsurveyed fractional districts so that final and complete maps of each could be constructed.
METHODS AND OBJECTS OF SURVEY. The methods of survey during the season were in part the same as those employed the previous year, modified to some extent as experience bad suggested. In addition to the determination of geodetic positions and general geographic features, the system of classifying the lands inaugurated in former years was continued, the object of this classification being to determine the extent and positiov of the irrigable lands, tiwber lands, mineral lands, and waste lands; the latter being composed of rugged mouutaius and desert plains. The practical importance of this classification if carefully made, is great, not only in presenting the in. formation desirable to those who wish to settle in the country, but also in the collection of facts necessary to intelligent legislation concerning these lands.
In the region embraced in this surrey a very small portion of the country can be redeemed by irrigation for agriculture, and no part of it can be cultivated without. It appears from the reports that less than one-half of one per cent. can be thus malle arailable. Especial care was given to the determination of the extent of such lands so as to exhibit their position on the maps. These irrigable lands and timberlands, together with some sipall districts of coal bearing lands are tbe ouly portions of the country that should be surveyed into townships and sections. Having in view economy and convenience in the linear surveys of this district, the geodetic points of the general geograpbic survey under my direction were carefully marked, that they might there. after be used as datum-points by the officers of the General Land Office.
Extensive coal fields exist in the region surveyed, but as in many other parts of the world they are of practical value at comparatively few places. The general cbaracteristics of these coal fields have been the subject of much investigation and some very interesting and valu. able results bave been reached; these will appear in the final reports. The quantity of available coal is practically inexbaustible aud tbe mines that can be economically worked are of great number.
Ip the Uinta Mountains silver and copper mines have been discovered and worked by private parties. The extent of these silver and copper bearing rocks was determined, but their value can be established only by extensive working.
GEOLOGICAL WORK BY MR. GILBERT. Mr. G. K. Gilbert devoted much of his time to the study of the structure of the Heury Mountains, of wbich enough had been learned in the preceding season to warrant the belief that they embodied a type of erup. tion hitherto unknown. The attention given to them bas been amply repaid by the elucidation of the manner of their constitution. They are volcanic, but their laras instead of finding vent at the surface of the ground and piling up copical mountains tbereupon in the usual manner ceased to rise while still several thousands of feet underground, and lifted the superincumbent strata so as to make for themselves deepseated subterranean reservoirs within which they congealed. Over each of these reservoirs the strata were arched, and a bill or mountain was lifted equal in magnitude to that which would bave been forined if the lava had risen to the surface; but the material of the bill was sandstone and shale instead of hard volcanic rock. Subsequent erosion bas carried away more or less completely the arching strata, and laid bare many of the intrusive masses. It has revealed also a system of reticulating dikes which go forth in all directions from the main masses, intersecting the sedimentary rocks. The lava masses, the dikes, and those portions of sbale and sandstone which have been metamorphosed by contact with the molten rock, are harder than the unaltered sedimentary strata which surround them, and yield to the agents of erosion more slowly. The wash of rain and streams by which the face of the surrounding country has been degraded has been resisted by these hard cores, and in virtue of their obduracy we have the Henry Mountains. The deposits of lava are not all in juxtaposition but are scattered in clusters, and each cluster has created a mountain, Mount Ellen consists of a score of individual lava masses; Mount Pennell and Mount Hillers each of one principal mass accompanied by several of minor importance; Mount Holmes of two masses; Mount Ellsworth of a single one, with many dikes and sheets. Each of the mountains is individual, topograpbically as well as structu. rally, and together they constitute a group of wountains, not a range.
Mr. Gilbert also made a valuable addition to our knowledge of structural geology by tracing tbrough Southern Utah the unconformity of the Tertiary upon the Cretaceous, which had previously beeu observed in other portions of the Plateau Province. He found an unconformity of dip amounting in some places to sixty degrees, and brought back sketches and photographs showing actual superposition and contact.
Before commencing the main work of the season, Mr. Gilbert made an excursion in search of the outlet of Lake Bonneville, the great fossil lake of Utab. During an epoch which was probably coincident with the Glacial epoch, the broad interior basin of Utah was covered by a great lake which overflowed its riin and sent an outlet to the ocean by way of the Columbia River. When the climate became gradually warmer and drier, the evaporation grew greater and the rainfall grew less, until finally the overflow ceased and the lake began to dry away and shrink within its shores ; to-day only Great Salt Lake, Utah Lake, and Sevier Lake remain, but high up on the mountain is carved the Bonneville Beach, a permanent record of the old flood tide. The search for the point of outlet was successful, and it was found at the north end of Cache Valley, a few miles beyond the boundary of Utab, in the Terri. tory of Idaho. The bed of the outflowing stream was traced for a number of miles. The beach lines were seen to run quite to the pass through which the channel was cut, but beyond, on the side of the drainage of the Columbia, no trace of them could be seen.
Of vo less interest was the discovery of a recent orographic movement at the western base of the Wasatch Range. A great fault runs along that base-one of the faults by wbich the mountains were produced. The block of the eartb's crust which lies to the westward of the fault plane was dropped down, or the block which lies to the eastward was lifted up, and from the eastward block subsequent erosion bas carved the range. Along the plane of ancient movement there has been a recent movement. The mountain has risen a little higher or the valley floor has dropped a little lower, and this so recently that the Bonneville floud is aucient in comparison.
GEOLOGICAL WORK BY CAPT. DUTTON. Capt. C. E. Dutton resumed bis study of the large area of igneons rocks in Southern Utab, in the vicinity of the Sevier River, and brought back additional information wbich he purposes employing in the prep. aration of a monograph of the entire tract. He worked out the structure of the coin ponent features and the approximate area of the erop. tions, and began the classification of the various lithologic members. The older outbreaks appear to be of early Tertiary Age (Eocene), and