17. APPENDIX D. REPORT OF G. K. GILBERT, GEOLOGICAL ASSISTANT.
DEAR SIR: In addition to the results of my own work, which was performed throughout the entire season of field duty, geological data were collected for limited periods by several gentlemen of the scientific corps. Mr. A. R. Marvine, serving in the double capacity of astronomer and geologist, was busied with geological examinations more especially during the latter half of the season, and will himself prepare the results of his labors for publication. Lieutenant D. A. Lyle, Dr. W. J. Hoffman, Mr.C.A. Ogden, and Mr.E.P. Austin have volunteered to contribute geological information in regard to some regions that I was unable to visit. Altogether the geological observations will be found to relate to about one-half of the lines of geographical exploration and survey. Keeping pace in our movements with rapidly executed geographical work, the geologists were unable to command the time necessary to the complete description of even the immediate line of travel, and the most we can claim to have accomplished is a reconnaissance of our field. Of this character essentially have been the labors of geologists attached to other exploring parties, and, indeed, the achievement of more thorough work in connection with exploration is neither possible nor, in every sense, desirable. I would not, of course, be understood that it will be unprofitable to make a careful and detailed survey of the geological structure and mineral resources of our whole territory, but that the first efficient step toward its accomplishment must be a cursory reconnaissance-a preliminary survey-with a view to obtain, at the earliest possible date, the broadest generalizations, the simplest and most comprehensive ideas in regard to the sequence and distribution of the rocks, to serve as a frame-work in which every later study of a locality or district may find its appropriate place. More than this, these primary generalizations, crude though they are, and subject to indefinite future modification, answer in the best feasible manner the most pressing demands of a region that must depend for its development on the understanding
Our field of operations has afforded us a view within the space of a single season of an unusual variety of geological features distributed over an immense area. Our southward progress, amounting to nearly seven hundred miles in a right line, was accomplished by a zigzag course that showed us a belt of country averaging one hundred miles in width, and expanded in one part to two hundred and fifty miles. Of a great portion of this region no geological description whatever has been written, but at several points it has been intersected by the lines of earlier geological exploration, the majority of which have crossed the country in an east and west direction. It has been our province to establish the connection, and measurably to fill out the intervals between them. The relations which our investigations sustain to those of Dr. Newberry, Dr. Antisell, M. Marcou and others, in Arizona; of Professor Whitney, Professor Blake and others, in California, and of the geologists of the Fortieth Parallel Survey, enable us to have a far better understanding of the phenomena presented by our field than would be possible if our work stood alone, and enhance in more than one way the value of the contribution we are able to make to the geological history of the continent.
Our work was facilitated, in a measure that can hardly be appreciated by persons unfamiliar with deserts, by the absence of trees and absence of soil that characterize the greater part of Nevada and Arizona. Not merely were rock exposures everywhere provided without search, but the view was in all directions unimpeded, and we could frequently see the limits of the different rocks beautifully delineated on the slopes of the distant mountains, revealing at a glance relations that in a fertile country would appear only as the results of extended and laborious investigation. This advantage, which we shared with all our collaborators in the interior of the continent, enabled us to obtain from what was within our reach no inconsiderable knowledge of what was merely in sight, and thus expand into a belt what might otherwise be a mere line of observation.
In the arrangement of the geological material for the final report, two needs are to be considered. By the resident, and by the traveler or geologist who shall follow in our steps, local details and exact localities, will be demanded; while the general reader, scientific or lay, will care only for the deductions that are of broadest application and such facts as are most important in their relations to the study of the continent. For this reason the report will be divided into two principal parts, of which the first (Chapters I-VIII) will record all observations of a local character in geographical, or rather itinerary order; and the second (Chapters IX-XVII) will contain a systematic arrangement and discussion of the results of our work.
‘‘CHAPTER I. Halleck Station to Ophir Cañon.-A portion of our travel in this interval was upon the belt traversed by the Fortieth Parallel Survey, and the latter part lay along a portion of the Toyabe Range, already described in the first published volume of the report of that corps.’’
‘‘CHAPTER II. Ophir Cañon to Pioche, and Silver Cañon to Big Pine.-This route carried us eastward over a succession of meridional ranges, nine or ten in number, and then obliquely back to a point one hundred and fifty miles farther south. With (apparently) two exceptions, these ranges consist of highly-inclined, stratified rocks, more or less metamorphosed, associated with granite, and flanked-in places even covered-by volcanic materials.’’
‘‘CHAPTER IV. Camp Mohave, via the Colorado River, to the mouth of Diamond Creek.-The cañons of Colorado, cutting down almost to the ocean level, give natural cross-sections of several ranges, and afford an opportunity to study on a grand scale the dislocations that accompany the upheaval of mountains. The geological section of the river bank exhibits at the east the undisturbed strata of the Colorado plateau with a thickness of one mile; toward the west, the dislocated masses of the same strata, forming a series of ridges with their upturned edges; and still farther, the granite nuclei and flanking schists and lavas of Virgin and Black Ranges.’’
‘‘CHAPTER V. Diamond Creek to the Triplets, in San Carlos Valley-Our course lay along the southern margin of the Colorado plateau, and in our repeated ascents and descents of its escarpment a number of sections were obtained, establishing the identity of the principal beds for a distance of three hundred miles.’’
‘‘"CHAPTER VII will contain some scattering observations between Independence, California, and Saint George, Utah, with a more continuous set of observations between the latter place and Camp Verde, Arizona Territory. From Saint George the course was south, the Grand Wash, lying between the eastern, precipitous face of the Colorado plateau and the Virgin Mountains, being followed, and the Colorado River crossed. Here we ascended from the granite to the lower bench of the plateau, and, traveling south and east near the edge, descended again to the granite near Truxton Springs. Granite, with some metamorphic rocks, predominates from here to Verde, forming, near the latter, the Black Hills, and being covered in some localities by lava fields and one or two isolated remnants of the plateau. The 'mineral' veins of this region occur in the granite and metamorphic formation.’’
‘‘"CHAPTER VIII. From Camp Verde to Tucson.-The valley of the Verde River, at the camp, indicates the line of demarkation between the granite of the Black Hills and the horizontal sedimentary strata of the Colorado plateau, which is here called the Black Mesa. We at once ascended and proceeded eastward to the Little Colorado, crossing the large basaltic mass which occupies the central area, rests upon the sedimentary rocks, and seems to have been continued somewhat farther north, in the vicinity of San Francisco Mountain. At the Little Colorado we turned south and east, crossed the Mogollon Mountains, and descended through the upper strata of the mesa to Camp Apache. The Mogollon, and probably the White Mountains farther east, are extended basaltic masses, resting on the mesa top in the same manner as San Francisco Mountain and adjacent lava masses. From Apache to Florence the course was southeast, passing through the lower sedimentary rocks of the mesa into the mountainous and diversified granitic region of Eastern Arizona, and out upon the deserts of the Gila River. Turning southeast, we remained on these to Tucson."’’
‘‘CHAPTER X. On the valleys of the Great Basin; their character dependent on origin, amount of filling, and present conditions.-The great majority are troughs between upheaved meridional ranges, partly filled by detritus (of subaqueous and subaerial deposition) from the adjacent mountains, and modified by floods of lava, that in numerous instances have connected parallel ridges and thus divided, more or less perfectly, the intervening valleys. In some of the lower areas, to which, in the gradual emergence of the continent, the sea had longest access, the valleys have been so completely filled as to connect with each other and constitute plains, through which the peaks (or the remnants) of the intervening ranges jut as ‘‘"lost mountains."’’’’
‘‘CHAPTER XI. On Erosion.-Besides the general discussion of the phenomena, so profusely displayed along the entire route, this chapter will contain notes on the pot-holes of the Colorado, and on the curious rock-sculpture executed by particles of sand driven by wind and by water. Here, too, will find place an account of some supposed drift-gravels, resting on the margin of the Colorado plateau near the Tonto Basin, in latitude 35° north.’’
‘‘CHAPTER XIII. On the distribution and age of the Sedimentary Rocks.-The fossiliferous beds, of which the age was determined by the expedition, range from the Primordial to the Upper Carboniferous, and rest unconformably on a series of highly crystalline schists, with associated granites. Fossils were obtained from about thirty-five localities, two thirds of which gave conclusive evidence of their geological horizon.’’
‘‘CHAPTER XV. On Volcanic Rocks and Mountains.-The entire field of our exploration has been the scene of prolonged, or recurrent, volcanic activity, reaching down to so recent a period that it would be rash to assert that it has even now finally ceased. There is no extended mountain range from the sides of which lavas have not flowed, and some are for long distances buried under the material that has found vent along their lines of fracture. From the Timpahute Range westward to the Amargosa, the eastern boundary of Death Valley, a distance of seventy-five miles in a direct line, we traveled entirely on volcanic material, and there is reason to believe that the field stretches northward an equal distance. In this and some other area, s north of the Colorado, rhyolitic and trachytic lavas predominate, and volcanic sands and tufas are conspicuous elements of the mass. Farther south, in the vicinity of the Colorado plateau, the latter are rarely seen, and basaltic lavas assume great prominence. The collection of volcanic products is large, and cannot fail, with study, to add something to the rapidly increasing store of facts in regard to the ordinal sequence of lavas.’’
‘‘CHAPTER XVI. Economic Geology.-To the difficult and important subject of the geological distribution of auriferous and argentiferous veins, and their relation to the mountain system and to the intruded rocks, we hope to contribute some facts of value.’’
‘‘Our report will record the occurrence and position of coal, salt, gypsum, and other economic minerals. The former has been discovered near Camp Apache in beds of Carboniferous age, and probably referable to the Coal Measures. The seam is of tolerable thickness, but has not been sufficiently developed to test its quality. It will probably prove to be of non-coking, bituminous coal.’’
‘‘In the geological collections, limited in extent by the circumstances of transportation, &c., great care was taken to represent the characteristic lithological features of the several geological provinces, the prevalent varieties of rock being studiously preferred to the locally exceptional.’’