field seems even yet scarcely explored. I prefer, therefore, to place before you some results of personal observation in a region in which I have worked for several years and in which I have become deeply interested.

The Absaroka Range lies along the east side of the Yellowstone Park. Several of its higher peaks and its long western spurs, sloping gradually toward the Park, lie within the national reservation. During several successive summers, while engaged in geological observations in the Park, I found it necessary to penetrate beyond its boundaries into the higher encircling mountains. My first excursion into the Absarokas was undertaken in the summer of 1885, and thereafter for several years I made long and protracted journeys into this rugged and at that time almost unknown region, studying its geology, and returning each year more and more profoundly impressed by its many marvels. In the year 1893, and again in 1897, the greater part of the summer was occupied in exploration of the wild recesses of the Absarokas.

The range, which lies wholly in the State of Wyoming, stretches from the Beartooth and Snowy ranges, on the north, southward to the Owl Mountains. In width it is less sharply defined, certain outlying plateaulike areas, such as Mirror and Two-Ocean plateaus, being separated from the main body by deep valleys. Geographically they may be considered as distinct physical features. Geologically, from their mode of occurrence and the nature of the rocks, they are intimately associated with the central mass, and for the purposes of this address they may be considered as forming a part of the Absaroka Range. As thus defined, the range measures 80 miles in length by 50 miles in width, covering an area of nearly 4,000 square miles.

From one end to the other the Absarokas present a high, imposing plateau, with ele

vations ranging from 10,000 to over 12,000 feet above sea level. This entire mass is made up almost exclusively of Tertiary igneous rocks. Near the northern flanks Archean schists and gneisses crop out from beneath the overlying rocks. Resting upon the Archean, upturned Paleozoic limestones. and sandstones having a considerable thickness come to the surface, and along the eastern borders of the range, exposed by erosion in the broader valleys, occur Cretaceous rocks. With these exceptions, the range consists of a vast accumulation of agglomerates, tuffs, lava flows and intrusive

masses.

Degradation of the mass has taken place on a grand scale. Vast quantities of volcanic ejectamenta have been removed from the summit, but no reliable data exist by which the amount can be estimated even approximately. All the higher portions have been sculptured by glacial ice. Enormous amphitheatres have been carved out of the loose agglomerates, and peaks, pinnacles, and relics of great tablelands testify in some measure to the forces of erosion. The plateau is scored by a complete network of deep valleys and gorges, which dissect it in every direction and lay bare the structure of the vast volcanic pile.

Nowhere in the northern Rocky Mountains do I know grander and more rugged scenery than can be found in the Absarokas. But few natural passes lead across the mountainous tract, and these are high and difficult to scale. For years the range stood as an impassable barrier to the earlier explorers in their attempts to reach the sources of the Yellowstone from the east; and even to-day the region is seldom penetrated to its inmost recesses except by those engaged in scientific exploration of the country, by the prospector in search of precious metals, or by a few adventurous sportsmen in pursuit of the big game of the Rockies. Much of this region is covered by a dense growth of

coniferous forest, and the greater part of the forests lying east of the Yellowstone Park belong to the Yellowstone timber reserve, the first of the forest reservations set aside by proclamation of the President under the Act of Congress approved March 1, 1891. Rightly to understand the true position of this volcanic area it is necessary to review briefly the geological history of the surrounding region before the piling-up of the eruptive material. The Absarokas are hemmed in, both to the north and to the south, by high ranges with approximately east and west trends. On the north are the Beartooth Mountains, presenting a broad elevated Archean mass culminating in some of the highest peaks to be found in Montana; while to the south are the Owl Mountains, consisting of an Archean nucleus capped and for the most part concealed by an arch of Paleozoic beds highly inclined along the outer edges. Between these two ranges lies a depressed basin, and resting unconformably upon the Archean are sediments of great thickness, derived in large part from the earlier continental areas.

These sediments, slowly deposited throughout a long period, represent nearly all the great divisions of Paleozoic and Mesozoic time. Beginning with the Cambrian, in their order of sequence, come the Silurian, Devonian, Trias, Jura and all the epochs of the Cretaceous recognized in Wyoming and Montana including the Dakota, Colorado, Montana and the Laramie standstone at the top, with its frequent fluctuations of sea level, foreshadowing changes in the development of the pre-existing continental

area.

With the close of the Laramie sandstone the long-continued deposition of Mesozoic and Paleozoic sediments finally came to an end. In this region unconformity of sediments by deposition has not as yet been recognized, and in this sense alone they may be said to be conformable from Middle

Cambrian time to the summit of the Laramie. Stupendous orogenic movements took place, and the surrounding country became one of mountain building on a grand scale, accompanied by plication, folding and faulting. The evidence all points in one direction-that this uplifting was contemporaneous in all the ranges of the northern Rocky Mountains. For this reason, and owing to its great geological significance, being one of the most important in Rocky Mountain geological history, the uplifting has been designated as the post-Laramie movement.

Along the west side of the Absarokas, and lying within the Yellowstone Park, extend north and south ridges of faulted and crumbled strata consisting mainly of highly inclined Cretaceous sandstone, the Laramie, nearly 10,000 feet above present sea level. From this ridge region eastward for fifty miles stretches this broad volcanic mass, finally dying out upon the plain over which the earliest lavas spread, resting on horizontal sandstones at an elevation of about 6,000 feet above sea level. After a very considerable erosion of the uplifted Mesozoic continental land area began the earliest of these volcanic eruptions, which later displayed such marvelous energy over this entire region of country, and which were closely related to the post-Laramie movement. This eruptive material, forcing its way upward, followed lines of least resistance along or near planes of faulting, or wherever the strain had been greatest upon the weakened strata.

The Absaroka Range was formed by the piling-up of successive accumulations of volcanic ejectamenta, with occasional interbedded flows of lava, burying everything beneath them to a depth of several thousand feet. Volcanic breccias, agglomerates and extrusive lavas, or those that have been poured out and cooled near the surface, constitute the bulk of the mountains.

It

These breccias and lavas were ejected from numerous fissures, vents and centers of explosive energy. Infinite detail as regards mineral composition and texture, and great complexity in mode of occurrence, may be observed. Viewed in a broad way and reduced to its simplest terms, the Absaroka Range consists of an uplifted volcanic region, presenting from one end to the other great uniformity, and even simplicity, in its main geological features. is essentially a dissected plateau, deeply trenched by incisive gorges, offering exposures varying from 2,000 to 5,000 feet of nearly horizontal or only slightly inclined lavas. To this there are, of course, some exceptions, as is natural in any volcanic region. Notwithstanding the varied and complex manifestations of the eruptive breccias from many sources of outflow, this entire body of extrusive material has been divided broadly into six epochs, based upon their relative age and general sequence of lavas. They represent, in the geological history of the mountains, as many distinct phases of volcanic eruption. Beginning with the earliest in the order of eruption, they have been designated as follows: early acid breccia, early basic breccia, early basalt sheets, late acid breccia, late basic breccia, late basalt sheets.

Briefly stated, the interpretation of this history, as I understand it, is somewhat as follows:

So far as is known, the oldest volcanic rocks recognized in the Absarokas consist of a series of eruptives made up almost entirely of fragmental material, usually light in color, varying from grayish white to purple. In mineral composition they range from hornblende-andesite to hornblendemica-andesite. Some of the siliceous varieties have developed phenocrysts of quartz in sufficient amount to be classed as dacites. These breccias appear to have been thrown out with violent explosive action from nu

merous centers, but from none of them was any large amount of material piled up; at least if it was thrown out it was subsequently worn down by atmospheric agencies. In no instance do they attain great elevation, the exposures being due to extensive erosion and deep trenching of narrow canyons. They are known only in the northern end of the range, and there in limited area, being buried beneath vast accumulations of still later material. These centers appear to be independent of later eruptions.

Overlying these acid breccias is a vast amount of volcanic ejectamenta, with here and there interbedded basaltic flows, the entire body having accumulated in many places to a height of several thousand feet. They occur far more widely distributed over the mountains than any other group of breccias, stretching both in its length and breadth from one end of the range to the other. They constitute nearly all the northern portion of the Absarokas, as well as the northeast corner of the Park. Unlike the early acid breccia, they are usually dark colored, owing to the amount of ferro-magnesian minerals present. The material consists largely of hornblende-pyroxeneandesite, proxene-andesite and basalt. Constant modifications and transitions occur, but over the entire area the prevailing rock is pyroxene-andesite, passing into slightly less basic rocks carrying hornblende on the one hand and into basaltic forms on the other. By far the greatest portion of this eruptive material is formed of coarse agglomerates, sombre in color, held together by varying amounts of cementing ash and silts of similar composition. The prevailing colors are black and brownish gray, while the finer silts and mud flows free from large bowlders are light brown, in strong contrast to the mass of the breccia.

It is difficult to describe in few words such volumes of volcanic material scattered

over broad fields and thrown out under varying conditions. Frequently these basic breccias present a rough and ropy surface, like ordinrry scoria irregularly heaped together, but the bulk of it indicates indistinct bedding. A tumultuous heaping-up of agglomerate by explosive action characterizes this breccia, which not infrequently carries andesitic and basaltic bowlders measuring 5 and 6 feet in length and often double that size. In one or two localities huge bowlders of crystalline gneisses and schists are also embedded in the lavas.

Scattered over the area occur the thin interbedded flows, apparently poured forth from numerous fissures and vents. These flows increased in frequency and thickness until finally massive outflows of basalt covered a considerable portion of the earlier series of breccias. Over how large a field they at one time may have extended cannot now be told, erosion having certainly removed them from large tracts, but they may never have been spread over extensive regions. It is somewhat curious that this continuous broad field of basalt has a northwest-and-southeast trend and stretches obliquely across the summit of the range from Mirror Plateau to Needle Mountain, whereas the body of the breccia in general has a north-and-south trend. The basalts lie upon the uneven surfaces of the breccia and occur piled up in a succession of flows, which in places near their sources have attained an aggregate thickness of 1,500 feet, although over large areas they measure about 1,000 feet, thinning out to a few hundred, while in certain places they appear to be wanting. Individual sheets range in thickness from 5 to 50 feet without showing any material change in the physical characters of successive flows. The greatest accumulation of flows appears to be along the trend of the basaltic body, thinning out both to the northeast and to the southwest, indicating that the eruptions had followed

a fissure or system of fissures. Of course, this can be said only in a general way, as basaltic outflows may occur anywhere along the range. As regards mineral composition, they are usually fine grained, with but few well-developed megascopic constituents, mainly augite, olivine and plagioclase. In chemical composition they show within restricted limits considerable variation, with accompanying changes in mineral development, analyses determining a large amount of the alkalies and a correspondingly low percentage of silica. Numbers of these flows have built up, from vents, rounded bosses of basaltic rocks characterized by a development of orthoclase, in several instances associated with leucite. They are the extrusive equivalents of intrusive rocks, designated as absarokites in distinction from normal basalts. Reference will be made to them later, in speaking of certain intrusive masses. So far as our present knowledge goes, they belong chiefly to this period of eruptions. Many of these individual sheets stretch out for long distances, but others show great lack of continuity, thinning and thickening in different directions and often overlapping one another, indicating numerous sources of eruption and varying force and duration of flows.

In their topograpic configuration the basalts stand out in marked contrast to the loosely compacted breccias, owing to great uniformity of flows and to differences in weathering. To these basalts the name early basalt sheets has been given, and they are here treated as a geological unit, since they mark a distinct period in the history of volcanic eruption. It is quite possible, and even probable, that they covered this entire region and were subsequently removed by erosion, but of this there is no direct evidence. If they did, the country must at one time have presented a gloomy, sombre field of basalt, poured forth in a molten condition after a long period of fragmental