while the capitals were of a brick-dust color, with excess of red oxide, and nearly as uniform in their granulation as fine millstone-grit.

The shape of these formations seemed, therefore, to turn on the comparative resistance to atmospheric influences possessed by their various parts. Many other indications, together with such reports as I could get from old settlers, and the experience of so acute a student as Mr. Pierce, led me to narrow down all the hypothetical agencies which might have produced them, to a single one,-air, in its chemical or mechanical operations, and usually in both. Water cannot be conceived of for an instant among the producing causes,- except in its vaporous dispersion through the atmosphere. Rain falls too seldom here (never in some localities of the mountains where these structures abound) to work much change in even the most friable rocks; besides, rain is a leveler, not a sculptor. No freshet from the mountains has topped these lofty hills since the creation of mankind; nor are they accessible to any water-course. But an all-sufficient denial to the hypothesis of water is the shape of the mimetic structures themselves. Water in motion is not easily deflected, and acts like a plane, not like a lathe. These skillfully turned cylinders, spindles, and cones point to a tool far more manageable, more readily carried around curved lines, and more minutely delicate.

This tool, in Colorado and other portions of the Rocky Mountain region, is none other than air or wind. This agent has never thus far received in our geological dynamics the importance it deserves. The atmosphere of this region is a chemical solvent, as energetic in some directions as it is inert in others. Its

oxygen is in a comparatively passive state. It will not rust iron exposed to it for years at a time, and the progress of pulmonary tubercle is often arrested in it at once. But over wide tracts it is charged with alkaline vapor, and, in virtue of that characteristic, possesses a power of decomposing the combinations of silex, which sometimes on our journey showed itself in ways quite surprising. I have seen large tracts in the heart of the range covered with crags and boulders belonging to a granite originally one of the most uniform and cohesive in texture among all our rocks, out of whose weather-worn faces the feldspar crystals could be scraped with the nail as easily as one would pick the seeds from a New Year's cake. Several large boulders seemed to have been corroded through and through. I kicked them to pieces as easily as the softest conglomerate.

The detritus resulting from such chemical decomposition has, during earlier ages, been brought down from the older rocks of the range in immense quantities, by the action of ice or floods. The whole region of the high divides we had been travelling from Denver, was thickly strown with such detritus; and in some cases, like the conical hills beneath the monuments, the ground was entirely composed of it. In its earliest stage, it was probably all one vast rubble bed, whose surface became gradually comminuted into sand, as on the yucca plains; or triturated and weather-beaten into a coherent layer, like that which forms the capitals of the columns.

The chemical energies of atmosphere having been exhausted in forming, with the aid of water, this superficially compacted drift-bed, mechanical causes began to operate, in the form of wind. Those who

think such an agency inadequate to the large and largely varied excavations which have taken place in the Colorado drift-bed, need only witness a whirlwind like those which it was my fortune to encounter both along the Platte and in the mountains, to make their minds entirely easy on the subject. There is no achievement of force beyond the capability of a Rocky Mountain tornado. It would take too long to investigate all the meteorological conditions which underlie this fact; but one abundant reason exists in the contour of the mountains, and their relative position with the Plains. The Plains, over their whole sandy surface, compose a vast radiator; discharging immense quantities of heat into the atmosphere during the entire sunny period of every day.

From dawn till night-fall the superjacent stratum of air undergoes constant rarefaction, and, as it ascends to meet the westerly current, is progressively carried into the higher mountain region adjacent. Here it parts with a portion of its caloric, but is pressed back by continuous rarifications from below, until with darkness the process stops, at a state of things like the following: an immense body of air condensed among the mountains, but every moment growing colder and heavier, a comparative vacuum existing immediately over the Plains below. The result is an immediate wind-cataract, falling from the height of about twenty thousand feet. But this fall does not make a straight plunge, like Niagara. It descends not over a precipice, but through a chasm. One characteristic of the Rocky Mountains is its system of vast indentations, cutting through from the top to the bottom of the range. Some of these take the form of funnels, others are deep, tortuous galle

ries, known as passes or cañons; but all have their openings toward the Plains. The descending masses of air fall into these funnels, or sinuous canals, as they slide down, concentrating themselves and acquiring a vertical motion. By the time they issue from the mouth of the gorge at the base of the range, they are gigantic augers, with a revolution faster than man's cunningest machinery, and a cutting edge of silex, obtained from the first sand-heap caught up by their fury. Thus armed with their own resistless motion, and an incisive thread of the hardest mineral next to the diamond, they sweep on over the Plains, to excavate, pull down, or carve into new forms whatever friable formation lies in their way. I can give no better idea of the efficiency of this instrument than by citing a few examples from actual experience. First, as to carrying capacity. That portion of the track between Denver and Pike's Peak which lies across the open Plains is every year repeatedly buried out of sight under gravel large enough to make it seem macadamized, blown from the foot-hills, a distance of several miles, by the ordinary winds of the region. It is no uncommon occurrence to see large trees in the path of the whirlwind torn up by the roots, and carried, revolving as they go, a distance of several miles into the Plains. Stones of many pounds' weight are sometimes served in the same way, seeming to be retained in the vertical whirl with as much ease as a cloud of dust or a splinter of wood.

Second, as to the force of the wind-auger. I myself have seen a hole bored into a Colorado sand-bluff, several feet deep, and of sufficient diameter to admit one's arm, by a small spiral current which rose on a

comparatively calm day, and without any general atmospheric perturbation. The work was done in a few seconds; and no machinery could have accomplished it more neatly. Mr. Pierce informed me of much larger excavations which he had seen effected with equal dispatch. But the account of his from which I gained my best idea of the exact composition and operation of the wind-and-silex auger, was to the effect that on a certain occasion, when he was stopping at a settler's cabin during the prevalence of one of these mountain whirlwinds, a spiral current, laden with sand-grains, impinged against one of the windowpanes, and, after a few moment's revolution, left it as perfect a piece of ground glass as could be made by a manufacturer of lamp-shades.

It is to the agency of this wind-and-silex auger that I ascribe all the mimetic formations of the Colorado foot-hills. Though a tool of tremendous force, it possesses a flexibility which enables it to accept any curved path; and this is an essential requisite of the instrument which can create such sculptures. It is a far more delicate tool than running water; for it acts by mechanical force alone, while water chemically decomposes the rocks whose surface it is abrading, and crumbles them to pieces while it is channeling their outsides. I consider the wind-and-silex auger the cleanest tool that Nature works with. It corresponds to man's highest advance in a similar direction, -the lathe for turning eccentric surfaces. The work that it does, no other agency could do; and we are thus indebted for one of the most characteristic features of our contemporary geology to a force scarcely noticed in its dynamics.

About four o'clock in the afternoon we came into a