The Mines of Colorado

CHAPTER XVI.

Geology and Geological History*- Classification of Minerals found in the Territory t.

GEOLOGY.

THE Granitic formation, which is the oldest, formed by the cooling of the original fiery fluid composing the globe, may be seen on and beyond the Snowy Range of the Rocky Mountains in various parts of Colorado; more abundantly on the western side of the Range than the eastern. In masses of true

granite, syenite, or porphyry, it makes its appearance on McClellan Mountain, in the Argentine silver district, where it is seen to have been thrust through younger formations to the prominent position that it occupies; on the west side of the Boulder Pass, where massive granitic ranges form the buttresses of the snowy sierra as we descend to the Middle Park; and on the western side of the Park, where it forms the grand mountain wall that encloses it.

Of Metamorphic rocks, gneiss is by far the most abundant, and most of the gold-bearing veins are

* Prepared by Prof. Wm. Denton of Boston. † Prepared by Mr. J. Alden Smith, of Trail Run, Colorado.

found in gneissoid rocks, though they are generally termed "granite" by the miners. Fine exposures may be seen near Black Hawk, the lines of stratification marking the mountain side, as stripes do the body of a zebra.

Resting upon the granite in the Middle Park, on the banks of Grand River, are exposures of conglomerate, probably of Silurian age, overlaid by sandstones and limestones, probably of Devonian age, and above this the coal measures of the Carboniferous formation, the only place in the Territory where true Carboniferous coal is known to existcoal that will coke.

Near the Sangre de Christo Pass, the granite is overlaid by slates and limestones probably of Silurian age, the limestones containing crinoidal fragments, but too small for the identification of the species. Farther north, the Three Tetons are composed of conglomerates, formed of pebbles, boulders, and large masses of gneiss, granite, mica-schist, and hornblende-schist, with gneissoid rocks, slate, and limestone on their flanks.

Rocks of the Permean age have been discovered on the Plains in the eastern part of Colorado, consisting principally of limestones, some of which abound with the characteristic fossils of this period.

The Cretaceous formation is well represented, especially along the base of the Mountains on the eastern side. The shells of the inoceramus are found in a limestone at Boulder City, baculites of large size in great abundance on the Platte, a few miles from Denver, while the limestones lying be

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tween Colorado City and Pueblo contain the inoceramus, scaphites, baculites, ammonites and other characteristic cretaceous fossils. These beds extend for a considerable distance to the eastward, and in wearing down under the action of atmospheric agencies, masses have been left in conical hills, looking like gigantic ant-hills; on these, fossils can be picked up in great abundance. Between Pueblo and the Sangre de Christo Pass, the teeth, spines and bones of fishes, principally of the genera Ptychodus and Lamna, so common in the cretaceous beds of England, are found in remarkable profusion. The neighborhood of Zan Hicklin's ranch on the Greenhorn River is the richest locality for fossils of this description that we ever saw.

In Eastern Colorado, coal measures are found of Cretaceous age, the shale and limestones accompanying them containing the characteristic forms of this period.

The Cretaceous formation is also well represented in the Middle Park by baculite beds and sandstone abounding with the scales of fishes.

The following shows the position of these beds as they occur on Chalk Creek, Middle Park:

Numbers 3, 4, and 5 are probably Cretaceous; the rest Tertiary.

From the disintegration of No. I proceed the agates and chalcedonies of the Middle Park. Where No's 1 and 2 are disintegrated, agates and fossil wood lie mixed together on the surface. The slabs of No. 3 are covered with the scales of cycloidal fishes, that is, fishes whose scales resemble those of the salmon and the trout. No. 5 we call Baculite beds, from the great number and large size of the baculites found in them.

The Tertiary formation has a remarkable development in Colorado. Its thickness as exposed on the western side of the Rocky Mountains, from the Parahlamoosh Range, which is composed of Tertiary lavas, to the junction of White and Green Rivers, is ten thousand feet. Included in this are coal measures, containing many thin veins of coal, beds of gypsum, thin beds of limestone, and above these, petroleum shales at least a thousand feet in thickness, abounding in fossil leaves and insects, the shales containing them occurring at points sixty miles apart; above them brown sandstones and conglomerates having a thickness of from twelve hundred to fifteen hundred feet and containing silicified wood, turtles, and bones and teeth of large mammals. The following is the order in which they lie in the valley of the White River: