the Wasatch Mountains on the west and the Green and Colorado Rivers on the east. Mr. Graves also made a complete plane-table sketch of the country surveyed, which, taken in connection with his angles for locations and perspective profile sketches, will enable him 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 unscalable cliffs, the 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 bodies of irrigable lands found are along the valleys of the Green and San Rafael Rivers. The only timber 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 confined 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 profile sketches were made by the aid of the orograph, a newly designed instrument that promises to be of great use iu topographic surveying. The work of Mr. Renshawe 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 the 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 3000

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 99° 30′, was by a former Secretary of the Interior divided into districts for surveying and mapping 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 in districts numbered 75, 85, 86, 95, 96, 104, and 105 (see accompanying map), the first five lying directly west of the region 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 earlier part of the work, before these districts were established by the Department, the operations of the Survey extended in an oblique direction. from northeast 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 position of the irrigable lands, timber lands, mineral lands, and waste lands; the latter being composed of rugged mountains and desert plains. The practical importance of this classification if carefully made, is great, not only in presenting the information 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 survey 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 made available. 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 timber lands, together with some small districts of coal bearing lands are the only 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 geographic survey under my direction were carefully marked, that they might thereafter 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 characteristics of these coal fields have been the subject of much investigation and some very interesting and valuable results have been reached; these will appear in the final reports. The quantity of available coal is practically inexhaustible and the mines that can be economically worked are of great number.

In 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 Henry Mountains, of which enough had been learned in the preceding season to warrant the belief that they embodied a type of eruption hitherto unknown. The attention given to them has been amply repaid by the elucidation of the manner of their constitution. They are volcanic, but their lavas instead of finding vent at the surface of the ground and piling up conical mountains thereupon 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 hill or mountain was lifted equal in magnitude to that which would have been formed if the lava had risen to the surface; but the material of the hill was sandstone and shale instead of hard volcanic rock. Subsequent erosion has 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 shale 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, topographically as well as structu rally, and together they constitute a group of mountains, not a range. Mr. Gilbert also made a valuable addition to our knowledge of structural geology by tracing through Southern Utah the unconformity of the Tertiary upon the Cretaceous, which had previously been 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 Utah. 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 rim 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 Territory 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 no 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 which the mountains were produced. The block of the earth'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 has 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 flood is ancient in comparison.

GEOLOGICAL WORK BY CAPT. DUTTON.

Capt. C. E. Dutton resumed his study of the large area of igneons rocks in Southern Utah, in the vicinity of the Sevier River, and brought back additional information which he purposes employing in the preparation of a monograph of the entire tract. He worked out the structure of the component features and the approximate area of the erup tions, and began the classification of the various lithologic members. The older outbreaks appear to be of early Tertiary Age (Eocene), and

to have been nearly continuous through a long period. The volcanic beds thus formed were subsequently traversed by great faults, and tables were uplifted with deep valleys between them, the structure thus produced conforming to the general type prevalent throughout the plateau country. The degradation of these long lofty tables gave rise to conglomerate beds of great extent and thickness, which are composed entirely of volcanic materials. Captain Dutton has compared the details and arrangement of these conglomerates with the alluvial beds now accumulating in great volume in the valleys out of the waste of the adjoining tables, and finds an agreement so close that he ascribes the same mode of origin to both. He also finds considerable metamorphism, not only in the underlying sedimentary beds (early Tertiary), but in the supposed conglomerate; and he thinks it must have occurred comparatively near the surface. The greater portion by far of the erupted rocks he classes as trachytes and trachydolerites. The rhyolitic varieties are of very limited occurrence, being found only in the vicinity of the Beaver or Tushar Range. In the southwestern part of the field (near Panguitch) extensive fields of basalt are found. Captain Dutton distinguishes two ages of the basalt: one prior to the development of the present structural features of the region, the other subsequent to it; the former being more properly dolerite or anamesite, the latter typical basalt.

ETHNOGRAPHIC WORK.

Under instructions from the Interior Department, my parties were also engaged in general ethnographic work in the Rocky Mountain Region. One of the special items in these instructions was the classifi cation of Indian tribes, such classification being not only of scientific interest but of great importance in the administration of Indian affairs. For the eastern portion of the United States this work had been accomplished, first by the unofficial labors of the Hon. Albert Gallatin, aud subsequently by the Hon. Henry R. Schoolcraft as an officer of the government; and some additions had been made by various persons for scientific purposes. This work was renewed by myself and pushed with all the energy possible with the funds at my command, and a large amount of material has been collected by myself and by members of my corps, and by residents in and travelers through the country. In addition to this a large amount had been collected by the Smithsonian Institution through various channels. That institution placed all this matter in my hands to be combined with my own collections.

PHOTOGRAPHIC WORK BY MR. HILLERS.

During a part of the season the photographer, Mr. J. K. Hillers, was attached to Prof. Thompson's party, making views for the illustration of the geological structure. Subsequently, in charge of a small party, he visited certain points in Northern Arizona to obtain some views in that region that were needed for the same purpose. He returned with a large amount of graphic material of great value.

BOTANY.

Mr. L. F. Ward, the botanist of the corps, assisted by several gentle. men of scientific ability in this department, was engaged during the entire year in the preparation of a "Report on the Botany of the Vailey of the Colorado," which is now nearly ready for publication.

OFFICE WORK OF 1876-77.

On the arrival of the parties from the field early last winter, work was promptly organized and pushed with all possible vigor through the winter and spring.

TOPOGRAPHIC WORK.

The first work of the topographers was the preparation of preliminary maps of the region surveyed during the season. These were constructed by making tracings of the plane-table sheets. In one month these maps were ready. In the mean time Professor Thompson and his mathematical assistants were engaged in computing the triangles and making the necessary adjustments for closure and determining the azimuths, latitudes, longitudes, and altitudes necessary for the construction of the final atlas sheets.

After completing the preliminary maps and following closely the progress of the mathematic work, the topographers engaged in the preparation of the final maps, and by the close of the office season, early in May, the whole work was made ready for the engraver.

The comparison of these final maps with the preliminary maps abovementioned was a rigorous test of the accuracy with which the topogra phers had done their work in the field and of the value of the methods and instruments employed. This was especially desirable from the fact that new methods and instruments were used, and while theoretically they appeared to be valuable, the test of experience was necessary for a final determination of their usefulness. The result exhibited the fact that the topographers could take the field with sheets upon which the primary triangulation was plotted and return with maps that would need so little readjustment after the final computations were made that it was scarcely perceptible ou the scale adopted for publication. And it was further demonstrated that a topographer in one field season could extend his work over an area of about five thousand square miles, and with all the accuracy necessary for the scale adopted by the Interior Department for the physical atlas of the Rocky Mountain Region, i. e., a scale of four miles to the inch.

As previously mentioned, this geographic work was under the immediate charge of Prof. A. H. Thompson, and his work is not more highly commended than it deserves in making the above statement.

INSTRUMENTS.

The base-measuring apparatus has been described in a previous report.

The theodolite used in the triangulation is of a new pattern, embrac ing a number of improvements demanded by the character of the work. So far as possible the number of parts has been reduced by casting in a single piece parts that are usually combined by screws. In this manner the liability to derangement incident to the vicissitudes of mountain work is greatly reduced. The telescope has been enlarged, as compared with the graduated circle, so as to make its defining power bear a greater proportion than usual to the refinement of graduation. The object glass has an aperture of two inches and a focal length of twenty. The horizontal circle is ten inches in diameter, and reads by double verniers to five seconds of arc. The vertical circle is five inches in diameter and reads to one minute. The instrument also embraces other improvements designed to secure greater stability, with ease aud rapidity in manipulation.