USA > Colorado > History of the State of Colorado, Volume II > Part 6
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The succeeding or Bridger beds are noted for containing abundant remains of the remarkable order, named by Marsh, the Dinocerata. These animals were of elephantine size, and related somewhat to the Coryphodon of the Wahsatch. They bore on the head three pairs of horn core-like prominences, which may have served for the attachment of horns, but are generally thought to have been simply covered with a layer of thick horny skin. In addition, they were armed with sharp, strong tusks, curving downward and backward. Though of great size and power, they possessed, like most Eocene mammals, an exceedingly small brain, and were consequently very low in intelligence. Great numbers of these animals inhabited the Green River basin, during the Bridger Stage, but disappeared entirely at its close; for which reason the horizon has been designated by Marsh the "Dinoceras Beds." Between the Green River and Bridger beds, King reports a slight non- conformity; indicating an interval marked by disturbances, which sufficed to elevate the basins above sources of brackish-water ; hence, during the Bridger Stage, sedimentation took place in fresh water, and the mammalian hordes again roamed the shores of the Eocene lakes.
The Uinta beds (Upper Eocene) are well exposed on Lower White River, where they consist of sandstones and brownish clays and
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marls. In Colorado there are soft strata of doubtful age, exposed on the Blue and Muddy Rivers in Middle Park, which Hayden has assigned provisionally to this group but which are probably of later age. Of the faunal remains from these beds, those of the Diplacodon, a genus of tapir-like mammals, are the most characteristic, which led to the horizon being designated the "Diplacodon beds."
The Eocene lake bed of the Huerfano basin, already mentioned, is of quite recent discovery, and the relation of its Eocene deposits to those of the great basins west of the Rocky Mountains, remains to be studied in detail. Among mammals the Tillodonts, which range from the Puerco up into the Bridger, are represented in the Huerfano beds. These peculiar mammals combine the general characters of Ungulates with the enormously powerful incisors common to Rodents. They were termed by Leidy, who first described them, "gnawing hogs." Carnivores, true Rodents and Lizards of the genus Glyptosaurus, like- wise existed in the Huerfano basin during the Eocene Epoch. Glypto- saurus includes certain species of extinct reptiles having the head and body covered with small tuberculated, enameled bony plates. So far as known, the Huerfano beds are the only fresh-water Eocene deposits lying east of the Rocky Mountains.
Throughout the Lower Tertiary, except in the beds of the Green River group, remains of numerous species of Ungulates and Carnivores, are common ; also of Monkeys and Rodents, many of the later species being more specialized types of allied forms already extinct.
One of the most prominent characters of early mammals and birds, as Marsh has shown, was the remarkably small size of the brain, when compared with that organ in existing species. There was, however, a notable increase in size during the remainder of the Tertiary, while Quaternary mammals had a brain capacity nearly equal to that of their modern allies.
The close of the Eocene in Colorado witnessed great changes in the topography of the land. The ranges were considerably elevated, and the strata on their flanks,-already more or less tilted by the con-
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tinental movement at the end of the Mesozoic,-thrown into great folds, either parallel or coincident with pre-existing lines of plication. At the same time, the region between the Rockies and the Wahsatch was elevated, and drained of its ancient lakes. East of the Front Range, the immense horizontal pressure, developed by the mountain- making movement, caused the formation of extensive areas of depres- sion in the adjacent plains country, and a corresponding elevation of the land further to the eastward. These depressions became the basins of the Miocene (Middle Tertiary) lakes; in which were deposited the sedi- ments now known, respectively, as the Monument Creek, and White River beds.
In the South Park region, at Florissant, there is a limited extent of beds believed, from the organic remains, to be intermediate between the Eocene and Miocene, or to belong rather to the epoch of the Oligocene. These beds abound in the remains of plants and insects, and have afforded several species of Fishes of the genus Amyzon, which has led to their being designated by Cope the "Amyzon beds."
The depression containing the White River beds lies mostly beyond the Colorado boundary, in Nebraska and Wyoming. The Monument Creek beds lie wholly in Colorado, covering a considerable area of country, east of the Front Range, between Denver and Colorado Springs. Both of these groups belong to the lower Miocene, with the probability that the Monument Creek beds may correspond to the lower part of the White River group; the horizon of the Brontotherium beds,-so named from the characteristic remains of a gigantic two- horned mammal, allied to the tapirs and to the Dinoceras of the Eocene.
The upper part of the White River beds does not contain Bronto- therium remains, but affords another genus equally characteristic, and restricted to that particular horizon ; viz., the Oreodon, an animal allied to the Hog, Deer and Camel. Hence, this portion of the White River group has been called the Oreodon beds. The Lower Miocene fauna, also, included new species of the Horse family, many new Carnivores,
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Rhinoceroses, Tapirs and Rodents, with the earliest of American Beavers.
Of the later Miocene beds none are represented in Colorado, these being confined to the Pacific coast, the Atlantic border, and the Gulf States.
In the interval preceding the opening of the Pliocene, or Upper Tertiary Epoch, additional dynamic movement occurred, other depres- sions were formed, and further elevation took place. But this was the last mountain-making movement of any importance; and, except that it has in places been deeply scored by erosion, the country has essentially the same orographic features now that it had in the Pliocene Epoch.
The Pliocene deposits of the West include the lower, or Pliohippus beds, and the upper, or Equus beds, so called from the characteristic remains of these genera of the Horse family. It has not yet been shown that either of the divisions is extensively exposed in Colorado, outside of the North Park basin, although certain limited deposits on the Huer- fano, and probably others in Eastern Colorado, belong to the Pliocene Epoch. The North Park beds cover a large area in the North Park depression, and on the Platte River in Southern Wyoming, but according to Hague, only develop a thickness of a few hundred feet.
The life of the Pliocene in this region may be inferred from the many species described from the Nebraska and Wyoming beds. These include several species of the Horse, Camel, Deer, Rhinoceroses, pow- erful Carnivores like the Tiger, an Elephant (Elephas Americanus) and the first Mastodon. The deposits of the Huerfano basin have recently afforded well-preserved remains of both the Horse and Camel. Many of these animals were of a size surpassing their living representatives, but were afterward overshadowed by the giants of the Quaternary.
The existence of man in California, during the Pliocene, has been maintained by no less an authority than Prof. Whitney, from the finding of flint implements, and human bones, in supposed Pliocene gravel. Others, however, who have examined the evidence express doubts of its authenticity. (Dana.)
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The termination of the Pliocene brings us to the close of the Tertiary Period. Mountain-making movement had ceased, but elevation of the Rocky Mountain region probably took place, to a limited extent, even after the Pliocene. From the close of the Marine Cretaceous to the close of the Tertiary the elevation of the land in Colorado, due to conti- nental movement alone, amounted to about 6,000 feet, while in the mountains this was supplemented by about 5,000 feet more, due to crumpling up of the strata. So in a few localities, notably on the head of the Rio Dolores, near Mount Wilson, and on the head of Crystal River, we find the Cretaceous beds tilted up on the flanks of the mount- ains to an elevation of 11,000 feet above sea-level.
QUATERNARY PERIOD.
This is the last chapter in our geological record, and its closing epoch brings our history up to the present time. The Quaternary in America begins with the great ice-age,-the Glacial Epoch. At that time all of Northern Europe, including the British Isles, together with the northern half of this continent, as far south as Ohio and Pennsyl- vania, was covered to a great depth with a continuous sheet of ice, whose duration in time was doubtless very great. The southern limit of the ice-field is marked by a deposit of boulder drift, called the "terminal moraine." Over all the country lying north of the moraine the rocks have been fluted and scratched by the steady southward march of the ice-stream.
With the final melting and breaking up of the North Polar Glacier, came the Champlain Epoch,-a time of great floods, and of the distri- bution of immense quantities of the material, which for ages had been carried forward by the ice. The Drift Epoch in Europe,-the equiv- alent of the Champlain in America,-was succeeded by the Second Glacial Epoch, of much shorter duration than the first. Evidence, by no means conclusive, is not wanting of the existence of a Second Glacial Epoch in America ; and by some geologists this is beginning to be regarded as a settled fact. The record left by the extinct glaciers of
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our own mountains, if anything, tends to support this view. Colorado lies far to the south of the great glacier limits on this meridian ; but the higher mountains, then as now, must have had a climate similar to less elevated regions far to the northward, or within the glacial limits. Hence one might conclude that a time of general glacial in the north would be represented by a time of local glaciation in our own mountains.
Evidence of the former existence of glaciers can be observed any- where in the mountain regions where the elevation exceeds 7,000 feet, and occasionally local glaciers have crept down into the valleys as low as 6,000 feet. The First Glacial Epoch may be represented by broad glaciated areas, often covered with heavy boulder-drift, such as we find on the White River Plateau, in the country just west of the Ragged Mountains, and in the upper San Miguel region,-areas which have since been deeply scored by transverse cañons. The Second Glacial Epoch may be represented by a later system of glaciers, which were confined to the principal valleys, and existed up to a very recent period, indeed, almost to the present day.
The Animas Valley glacier was, doubtless, the longest of the local ice-streams, and must have had a length of fully sixty miles. Huge boulders of granite, transported by the glacier, are found some distance below Durango. Terminal moraines, or ridges of boulders stretching across the valley, mark the halting places in its final retreat back to the snow-fields. One such moraine, formed by two parallel ridges of drift, crosses the valley at Animas City. All the valleys in the San Juan Mountains, and in the Elk Mountain region, afford indisputable evidence of the existence of glaciers at no very distant period ; when the mean annual temperature was probably lower, and the average precipitation greater than at present.
The drift deposits of Colorado are, in places, quite extensive, but have not yet been studied outside of the Denver basin, and the assignment of any portion of them to the Champlain Epoch is therefore provisional.
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It seems probable that much of the boulder-drift, covering certain elevated areas of the State, is truly morainal in character and may ante- date the age of the Champlain, provided the existence of the First Glacial Epoch, in the Rocky Mountains, shall be clearly established. On the other hand, the coarse drift of the mountain valleys can only be considered as the morainal material of the more recent local glaciers that has been subjected to fluvial reassortment; consequently all such drift properly belongs to the present era. The drift deposits scattered over the plains, or underlying the loess-like accumulations of the great valleys, are really the only beds which may be regarded as the probable equivalent of the Champlain. The löess-like deposits, often of consid- erable thickness, which are frequently met with on the plains and in the valleys, should no doubt be referred to the very uppermost Quaternary, when subaerial degradation and corrosion furnished material which could be distributed by æolian agencies.
The life of the early Middle Quaternary differed from the modern in many important particulars. The Carnivores, Ungulates, Probo- cidians, Edentates and Rodents were all of the most gigantic size; and their remains, which are so abundant in the drift of Europe and America, are found, on the former continent, associated with the remains and rough stone implements of Palæolithic Man. In America, the evidence of man's existence in the Champlain Epoch is confined to certain remains stated to have been found in the lava-covered auriferous drift of California, concerning the age of which there is some doubt, and they may belong to the Pliocene Tertiary. The finding by Mr. Belt, a well known English geologist, of a human skull in drift, of probable Quaternary Age, exposed in a railway cut near Argo, may be cited as indicating the bare possibility of man's existence in Colorado during the Champlain Epoch. The death of Mr. Belt soon after, and the want of any complete published statement by him, renders it impossible now to judge of the value of the discovery. Assuming, however, that the skull was found in Quaternary drift, the limited thickness of the deposit in the Denver basin, and in the locality cited, would place the horizon of
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the find within the reach of burrowing animals, and through their instrumentality remains of all kinds might be carried down into the drift, and in course of time, so far as the eye could discover, appear to be in place there.
The remains of Quaternary mammals, known to have been found in Colorado, include species of the Mammoth, Camel, Rhinoceros, and Horse, all of gigantic size; indicating that the life was identical with that of the remainder of the continent. All of these species except the Mammoth, which had already appeared in the Pliocene, probably invaded the country at the end of the First Glacial Epoch, but disap- peared at the beginning of the Second Glacial Epoch, which was fol- lowed by the invasion of existing species.
Throughout the Cenozoic, the fauna of each succeeding stage had its allies in the more generalized fauna of the preceding stage; and the tendency was strongly toward the development of more perfect types with greater brain capacity and higher intelligence. But between the mammals of the lowest known Tertiary and those of the preceding epoch (Upper Laramie) there is a great zoological break. The Lar- amie mammals have their affinities among the earlier marsupials of the Jurassic. The large number of Laramie species brought to light by the recent investigations of Marsh, are nearly allied to the ancient types, and fail to exhibit any anatomical characters foreshadowing the highly organized mammals which suddenly appeared in countless num- bers in the early Eocene of Colorado and Wyoming.
This is one of the most surprising gaps in the whole range of geological history. Yet such a break in the continuity of the record might, indeed, result from the great change of conditions effected by the continental revolution. It is also within the range of probabilities that, in the comparatively unexplored portions of the West, especially the Northwest, we may find transition beds between the Laramie and Eocene, and in them the remains of the long-sought progenitors of the Eocene hordes.
There are certain phases of geological development which cannot
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well be treated chronologically, and at the same time comprehensively. Belonging to this category are the ancient eruptions and ore deposits ; the consideration of which, for the above reason, has been referred to the last part of this chapter.
ERUPTIVE ROCKS AND ERUPTIONS.
During the mountain-making period, the entire Cordillerian region of the West was the scene of great igneous activity, and of eruptive outbursts in magnitude unsurpassed in the world's history. This activity was manifested on a grand scale in Colorado, especially in the south- western portion, and it is safe to say that one-seventh the area of the State is covered with eruptive rocks. They are found breaking through metamorphic and sedimentary strata of all ages from the Archæan to the Tertiary inclusive. The principal eruptions took place in the early part. of the latter period, a few being of preceding, and others of somewhat later age; although few can be cited more recent than the Miocene, and only one can be referred to Post-Tertiary times.
The kinds of eruptive rocks found in Colorado, not including the numerous intermediate varieties, are the following, based on the modern classification :
Porphyry: A crystalline, or granular, aggregate of orthoclase (potash-feldspar), usually with some plagioclase (soda-lime-feldspars) and quartz. Other minerals may appear and give rise to varieties, for instance, hornblende-porphyry. The Colorado porphyries are mainly quartz porphyries.
Trachyte: Differs from porphyry in containing the variety of orthoclase called sanidine, and in having a more or less glassy or felsitic groundmass.
Rhyolite : Consisting of glass alone (pearlite and obsidian), or of glass containing a relatively small number of quartz and sanidine crystals (liparite), or of glass containing a relatively large number of the same crystals as compared with the groundmass (nevadite).
Diorite: A crystalline aggregate, of like-sized grains, of plagi-
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oclase, with either hornblende, angite, enstatite (hypersthene), biotite or quartz. The term is usually qualified by prefixing the name of the principal constituent mineral, as quartz-diorite, mica-diorite, quartz-mica- diorite, etc.
Porphyrite: Corresponds essentially to diorite, but with one or more of the minerals conspicuously (porphyritically) developed as crys- tals, in the crystalline or granular groundmass.
Andesite: Differs from porphyrite, mainly in the groundmass, which is more or less glassy or felsitic.
All the above rocks have a high percentage of silica, and for this reason are termed "acidic ;" the five next succeeding contain a compar- atively low percentage of silica, and are termed " basic."
Basalt: Contains plagioclase and angite, frequently with olivine, in a felsitic or glassy groundmass.
Dolerite: Corresponds essentially to basalt, but has a granular or wholly crystalline groundmass.
Tephrite: Mainly plagioclase with nepheline or leucite, sometimes with both, and generally with other minerals as accessories ; in a partly felsitic or glassy groundmass.
Phonolite: Principally orthoclase and nepheline, with conspicuous crystals of sanidine, in a more or less felsitic groundmass.
Peridotite: Consists mainly of olivine, but varieties contain horn- blende, angite, etc.
Eruptive Breccia: Contains fragments of eruptive or other rocks, embedded in an originally plastic eruptive matrix.
Tufa: Consolidated ejectamenta from live.
Volcanic Ash: Consists of fine particles of volcanic glass or dust, not consolidated.
So far as known, phonolite and peridotite are extremely rare, only one occurrence of each having been announced by Dr. Cross from Col- orado localities. The same authority reports as probably nepheline- tephrite, certain eruptive rocks from the Elk Head Mountains, collected by Mr. F. F. Chisholm. Trachyte occurs at Silver Cliff, in the Mos-
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quito Range, and probably at Del Norte, but is one of the least common of Colorado eruptives. Limited accumulations of volcanic ash are found in the Pliocene beds of the Huerfano basin. The remaining eruptives are of frequent occurrence.
The most recent manifestation of igneous activity in Colorado, was the outburst of scoriaceous lava on Eagle River, near Dotsero ; which, according to Prof. Lakes, was poured from a vent situated in a small amphitheater about three or four miles north of the river. The flow is quite modern in appearance, suggesting the probability that it may have been erupted during the historic period.
The most recent of the great Tertiary eruptions that occurred in Colorado are represented by the vesicular basalts of Grand Mesa, and of the country lying between Roaring Fork and Eagle River ; likewise, certain small masses on the Rio Grande near the New Mexico line. These were preceded by overflows of more compact basalts, dolerites, and allied basic rocks, which were, in turn, preceded by rhyolites, andesites and andesitic breccias ; the whole corresponding to a series of grand eruptions, extending back to the early Tertiary. The older basalts and dolerites are represented by the Fisher's Peak overflow; by the overflows of the White River Plateau and Elk Head Mountains; by certain occurrences in the North Park and Middle Park regions, and on the Piney ; and by the small Table Mountain overflow, near Golden. The andesites, breccias and rhyolites are well represented in the San Juan region ; in the district around Silver Cliff ; in the Mosquito Range, and Ten Mile districts. Rhyolite containing garnet and topaz, occurs near Nathrop; while the tufaceous rhyolite of Castle Rock is well known, being much used for buildings in Denver.
The time of greatest igneous activity probably corresponded to the period of greatest mountain-making movement,-that is, about the end of the Eocene Epoch. Belonging to this period are certain porphyries, diorites and porphyrites, which do not occur as overflows, but appear as masses of mountain dimensions, tilting up, or arching the flanking strata ; and sometimes spreading, as huge lens-shaped bodies, laterally beneath
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them. The eruptive cores of the Ragged Mountains, Mount Carbon, Mount Gothic, Crested Butte, Snow Mass Mountain, Mount Sopris, La Plata Mountain, Ute Peak, Spanish Peaks, Veta Mountain and Badito Cone, with several others, are of this character. The same rocks often occur as dykes traversing sedimentary strata, or as thick sheets intruded conformably with them. All of these forms are well illustrated in the Spanish Peaks region.
Eruptive rocks have directly, or indirectly, played an important part in the production of the several varieties of Colorado coals ; the change from lignite to semi-coking or coking-coal, or to anthracite, being in each case clearly traceable to the heat directly radiated by eruptive masses, or indirectly applied through the medium of hot water. The association of these rocks with metalliferous veins can be best considered in connection with the next subject.
ORE DEPOSITS.
Certain eruptives have apparently greatly influenced the conditions attending the genesis of many ore-bodies. The analyses of Leadville rocks by Hillebrand, showed them to contain very minute quantities of the precious metals and lead, only determinable when considerable of the material was subjected to analysis. Nearly all the eruptives of Sum- mit district in Rio Grande County, can be made to yield appreciable quantities of gold by fire assay. In Europe, where many rocks of this class have been specially analyzed for metals, small quantities of the latter are usually found as an ingredient of one or more of the basic silicates. There is no reason why many Colorado eruptives should not yield like results when fully investigated. Masses of mountain dimen- sions, even though containing metals in mere traces, are capable of enriching the material of veins traversing them, to an extent that will make the latter economically valuable. In the eruptive rocks, and probably also in the crystalline schists, we have all the elements required in the formation of productive veins, under conditions favorable to the secretion of the disseminated contents. In but few cases does it appear necessary to call in the aid of solutions ascending from deep-seated
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sources, in order to account for the origin of a particular ore-body; for admitting that metals were originally brought up from great depths, we must still regard the eruptive rocks as the most reasonable medium of translocation, since they have emanated from a source more deep-seated than it would be possible for circulating waters to attain. The theory of lateral secretion, now very generally advocated, derives additional support from the frequent association of metal veins with eruptive rocks of certain types; while other types again are seldom known to contain important ore-bodies.
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