USA > Colorado > History of the State of Colorado, Volume II > Part 2
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The ancient granites and gneisses do not partake of the characters common to rocks which have resulted from lava consolidation ; on the contrary they belong, more properly, to another large class of rocks, apparently produced by the slow crystallization (metamorphism) of sed- imentary deposits, through long-continued subjection to elevated tem- peratures in presence of water, and probably great pressure. These are termed metamorphic rocks, and the granite series may be regarded as the extreme term of such metamorphism, the varieties being, simply, aggregations of easily recognized, definite mineral species. Evidently, the origin of this class of rocks must have been subsequent to the time of first consolidation, or what may be termed the first stage of rock for- mation, and should rather be referred to the second stage, during which, the hydrothermal conditions necessary to metamorphism, first came into existence.
Following the first superficial consolidation, came long ages of con- stant but gradual cooling, accompanied by slow thickening of the solid crust, until finally the temperature of the surface was reduced to the point at which the condensation of water became possible. This was the beginning of the second stage of rock formation. Degradation and sculpturing of the surface began with the advent of water, and, considering the conditions then existing, the effects must have been stupendous ; for the temperature of the earliest seas, probably exceeded the boiling point, while from an atmosphere saturated with steam, and acid vapors, ceaseless torrents of hot rain were precipitated. The sur-
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HISTORY OF COLORADO.
face of the globe was one vast cauldron. Water was then the all- powerful agent in the work of breaking up, and reconstructing, the material of the original crust. Nor was its action restricted to rapid mechanical erosion. In its heated and highly mineralized condition, it was capable of bringing about wide-spread chemical changes, not only in the nature of the decomposition but in the reorganization of material into mineral aggregates. The water of the ancient seas thus heated, and charged with mineral substances, was doubtless one of the principal factors involved in the metamorphism of the rocks of the earlier series, to which the greater part, if not the whole, of the granite rocks of the globe in all probability belong.
Some geologists even maintain that the hot, saturated water of the primitive seas was the principal agent in the formation of granite and allied rocks. This view, however, has not received much support, the most general conclusion being that complete metamorphism could only result from the subsidence of sediments to depths where the tem- perature was sufficiently high to induce chemical action. The latter view is, no doubt, most in keeping with observed facts, and may be largely true, yet it will hardly account for the universal and complete meta- morphism of the oldest sediments, or, to speak more plainly, of the ex- istence everywhere of a granite substructure.
The question of the origin of granite is still an open one, conse- quently, its discussion would exceed the scope of this work. It is merely necessary to state here that, beyond doubt, the process of granite formation required the presence of water at comparatively high temperatures, and under considerable pressure, and that the water of the ancient seas was active in bringing about consolidation of the earlier sedimentary accumulations, or was even capable of transforming them to some extent into crystalline aggregates,-that is, of inducing the first stages of metamorphism.
Considering how infinitely prolonged must have been the time during which the above causes were in operation, and also their com- paratively great activity, it is not surprising that the first-formed crust
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HISTORY OF COLORADO.
has-at least as regards its original character-long ago disappeared, or become deeply buried under vast detrital accumulations long since transformed into highly crystalline granites and gneisses.
The long period of time associated with these changes, probably greater than all subsequent time up to the present, is usually termed by geologists the Archæan Era. The granite rocks of Colorado and other parts of the world, were formed during Archaan times; conse- quently, so far as we can judge, the first important emergence of the land did not take place until toward its close, and the earliest land areas, as we know them, were really such as existed during the first period of the succeeding or Paleozoic Era. Hence, our geological history really begins with the dawn of the Paleozoic, whose successive periods were so many stages in the progressive development of the ancient systems of terrestrial life.
It begins with the earliest record of the actual existence of life, as clearly demonstrated by fossil remains. Regarding the previous exist- ence of life we possess no absolute proof, although there is sufficient evidence, of an indirect nature, to warrant the conclusion that to- ward the latter part of the Archæan, the lower forms of organisms were abundantly represented.
The form and extent of the Colorado land-surface, at the opening of the Palæozoic, can only be outlined in a general way. The Archæan areas, as defined by Hayden, simply represent the Archæan rocks now exposed, and not the dry land actually existing at the beginning of Palæozoic times.
Beyond question the dry land of that period must have been much more extensive than at any subsequent time in Palæozoic history ; for throughout this era there was a gradual subsidence during which an enormous thickness of sediments, derived from the exposed areas through erosion, was deposited. Thus a large part of the first dry land was again slowly submerged, and became deeply buried under the con- stantly accumulating sediments resulting from its own degradation.
The map at the beginning of this chapter represents the probable
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HISTORY OF COLORADO.
form and extent, of the Colorado land-surface at the dawn of the Palæozoic. The shore contour is merely an approximation, and the area indicated was very much less at the beginning of the succeeding era ; while the two main islands, although probably separated during the Silurian, were joined by a narrow isthmus toward the north during or preceding the Carboniferous Age.
Judging from the extent and thickness of stratified rocks, which could only be derived from this land-surface, through its denudation, the mean elevation must have been very great ; surpassing anything known of like areas at the present day. One can only imagine the landscape of these ancient islands,-if indeed it was ever visible through the dense mists of the humid atmosphere,-to have been marked by extremely high mountains, and very deep gorges, with a general surface rough, water-scored, rocky, and utterly devoid of animal or vegetable life.
All the divisions of the Paleozoic, from the Cambrian to the Permo-Carboniferous inclusive, are probably represented, to a greater or less extent, in the geological sections of Colorado. The lower divis- ions however, appear to be wanting in characters whereby they can be specifically identified or defined, and with the possible exception of the Cambrian have nowhere a thickness approaching that developed by cor- responding Palæozoic strata in the region of the Appalachians. This statement is true so far as regards this part of the Rocky Mountains ; but the development in the Wahsatch is very much greater, the total thickness of Paleozoic strata being reported by Clarence King at 32,000 feet. The Paleozoic rocks of Northeastern Colorado, -- that is, those exposed along the base of the Front Range,-probably do not exceed 1,000 feet in thickness. In Southeastern Colorado, along the Sangre de Cristo Range, they probably attain a thickness of 4,000 feet, pos- sibly more, since the existence of transitional beds-consisting of an enormous thickness of sandstones-renders it difficult to determine where the Paleozoic ends, and the Mesozoic begins. In the Mosquito Range, according to S. F. Emmons, the Palæozoic rocks develop a total thickness of 4,000 feet; while in the San Juan Mountains of
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HISTORY OF COLORADO.
Southwestern Colorado, the corresponding rocks have a maximum thick- ness of not less than 15,000 feet. All the Palæozoic strata were sub- sequently buried under more recent sediments, the latter generally overlapping along the shore-line; and since the former were deposited on a sloping surface, they were thinner along the old shores than else- where. From which it follows that the thickness exposed will be greatest in localities where the old shore-deposits have been deeply eroded. In describing the Palæozoic beds of Colorado it will be best to take them in their occurring order, beginning with the lowest in the scale.
SILURIAN SYSTEM. CAMBRIAN OR PRIMORDIAL PERIOD.
Of the Cambrian rocks, or what have been provisionally assigned to this period, the greatest thickness developed is in the San Juan Moun- tains in Ouray County. They consist of quartzites, slates, and quartz conglomerates, aggregating from 10,000 to 12,000 feet in thickness, exposed in the gorge of the Uncompahgre River above Ouray. This great development of Cambrian strata, nearly equal to that observed in the Wahsatch Mountains, is altogether local, probably owing to the fact that the sedimentary beds have been more deeply eroded on the Un- compahgre than on any stream draining the western slope of the San Juan Mountains, except the Rio Las Animas. On the branches of the Rio San Miguel erosion has not even exposed the uppermost of the Palæozoic strata ; while on the Rio Dolores the Cambrian quartzite is barely exposed in the mouth of Silver Creek, by the erosion of a great anticlinal uplift cut by the river. Rocks, part of which may be Cam- brian, are exposed in the Needle Mountains south of the Rio Las Animas. These three exposures probably belong to one and the same series of beds extending beneath, and hidden by overlapping strata of more recent age. The remaining Cambrian exposures of Colorado, so far as known, are by comparison quite insignificant. Emmons reports only 200 feet in thickness as being developed in the Mosquito Range, and but 50 feet in Manitou Park. Quartzites which Hayden refers to the Silurian, but which may contain some Cambrian, are exposed on
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HISTORY OF COLORADO.
Grand River between Glenwood Springs and Dotsero, and extending northward, cover a portion of the White River Plateau.
Regarding the life of this period in Colorado little is yet known, and at present its character can only be inferred from the remains abounding in other regions, and which conclusively show the world-wide distribution of the dominant types. The Primordial rocks of the globe contain the oldest known faunal remains, which are represented in Europe, and different parts of Eastern North America, notably also in Nevada and Utah, largely by Crustaceans (Trilobites) belonging to genera of Olenellus, Paradoxides, Olenus, etc. These are associated with Mollusks, representing species of Brachiopods, Gastropods, Cepha- lopods and Pteropods. The only plants were sea-weeds. Marine worms, and sponges, also made their appearance in this period, and Echinoderms toward its close.
The dominant forms were Crustaceans, which were comparatively numerous, and the individuals of remarkably large size. All of the species, and several of the genera, became extinct at the end of the Cambrian. The oldest rocks of this period contain the remains of a genus of Crustaceans called Olenellus, which, with certain associated forms are collectively termed the Olenellus fauna, and the horizon at which they are found, the Olenellus zone. Usually, where this zone has been identified in the West, it is underlaid by a considerable thick- ness of Pre-Cambrian stratified rocks; hence, the probability that part of our Colorado Cambrian may be Pre-Cambrian, or Algonkian as defined by Walcott. It is interesting, in connection with this oldest of known faunas, to note the high degree of perfection already attained by animal life. Contrary to what might have been expected, we find the Crustaceans among the largest of the kind ever known; while among Mollusks several of the grand divisions of the present time were well represented. These facts lead one to conclude that, between this period and the Archæan, there existed long ages of organic devel- opment of which the record is still wanting, and during which these
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HISTORY OF COLORADO.
highly organized types were slowly evolved from the primitive forms indirectly shown to have swarmed in late Archæan seas.
The presence of a typical Cambrian fauna in Utah, Nevada, and Arizona, in rocks deposited in the bed of the same sea, renders it highly probable that similar forms abounded along the Cambrian shores of Colorado, notwithstanding the scarcity of fossils in the few localities where they have been searched for.
The remaining Silurian rocks of the West have nowhere attained a development approaching that of the underlying Primordial.
In Middle Nevada, according to King, the Silurian, exclusive of the Cambrian and Quebec, has a total thickness of 2,000 feet; and, according to the same authority, but half this thickness is exposed in the Wahsatch Mountains. In Colorado but 200 feet is reported by Emmons in the Mosquito Range. It has been identified in several localities in Nevada, Utah, Arizona and New Mexico by characteristic fossils, mostly of the age of the Canadian and Trenton Periods of the Lower Silurian (Ordovician).
The rocks are usually limestones which, in Colorado, rarely contain fossils. The limestone exposed just above Ouray on the Uncompahgre, and which underlies, uncomformably, the strata of the Carboniferous there so conspicuously developed, probably belongs to the Post- Cambrian portion of the Silurian, judging from the lithological suc- cession observed elsewhere in the West. Rocks of corresponding age, cover a considerable part of the White River Plateau.
Among the rocks frequently exposed is a pinkish colored sandstone containing numerous worm-burrows, in some places normal to the planes of bedding like the Scolithus of the Potsdam sandstone. Silurian strata, of limited thickness, are often present at the base of the upturned sedi- mentaries, bordering the several Archæan areas, but are not deemed of sufficient importance to demand special mention.
While the evidence of life afforded by the Silurian rocks of Colorado, is of the most meager description, it does not follow that the conditions were altogether unfavorable for its existence,-for, no doubt,
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HISTORY OF COLORADO.
subsequent alteration of the rocks has had much to do with the oblit- eration of the life-record. In the Appalachian region, and in Europe, the strata of this age afford a marine fauna rich in species. Land-plants, represented mostly by Club-mosses, first made their appearance at about the middle of the age, and in Europe, Vertebrates (fishes) also. These were the precursors of the life which attained such an expansion during the remainder of the Paleozoic.
DEVONIAN SYSTEM.
This system of rocks, so well defined in Europe and Eastern North America, is, so far as known, barely represented in Colorado. Pinkish, or red-colored, sandstones, bordering the Archæan, near Canon City, appear to contain characteristic Devonian fossils.
The upper part of the beds referred to the Silurian may really belong to the Devonian ; a question which has not yet been decided, owing to the absence of palæontological evidence. King reports the Devonian quartzite and conglomerate as 2,400 feet thick in the Wahsatch Mountains, and 2,000 feet thick in Middle Nevada. The quartzites and conglomerates exposed around Treasury Mountain, in Gunnison County, may, on the ground of lithological similarity and order of succession, be referred to the Devonian, and for the same reasons the underlying limestones are probably Silurian.
The remarks made with regard to the paucity of organic remains in the Silurian rocks of Colorado are equally applicable to the Devonian rocks. In other parts of the world they abound in fossils, indicating that the earth teemed not only with animal but with vegetable life, and with forms much higher in the scale of development than are found to have existed in the preceding age. The land was clothed for the first time with forests of coniferous trees,-with Lepidodendrons, Sigillaria, Calamites and ferns ; while the seas swarmed with Ganoid and Placoid fishes, covered with bony plates or scales, and possessing characters which allied them in part to the reptiles. True fishes (Teleosts) and true reptiles were, however, yet unknown; nor did the Devonian veg-
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HISTORY OF COLORADO.
etation reach the exuberant growth which characterized the forests of the succeeding age.
CARBONIFEROUS SYSTEM.
The rocks of this system in Colorado are better defined and more evenly distributed than those of either the Silurian or Devonian systems. They are usually separated into three divisions, corresponding to the three periods of this age, viz .: The Sub-Carboniferous, the Car- boniferous proper, or coal measures, and the Permo-Carboniferous. In the Wahsatch Mountains and in Middle Nevada the strata of the three periods aggregate about 15,000 feet, of which about one-half is lime- stone. In Colorado the Carboniferous varies in thickness from a few hundred to nearly 5,000 feet, according to the distance of the exposures from the old shore-line,-or, in other words, according to the amount of erosion. The line of demarkation is generally clearly defined at the base, owing to angular non-conformity with the underlying rocks. At the top of the Carboniferous series it is rarely possible to separate defi- nitely the occasionally fossiliferous sandstones of the Permo-Carbonif- erous from the non-fossiliferous sandstones at the base of the overlying Mesozoic,-that is, the two blend insensibly into one another. The same absence of demarkation is generally observed at the junction of the Permo-Carboniferous with the coal measures. Between the latter and the Sub-Carboniferous the line of separation is usually well defined.
Throughout the Carboniferous exposures of Colorado there exists a common and easily recognized lithological similarity. The Sub-Car- boniferous consists mainly of limestone ; the coal measures of gypsiferous clays and shales, with more or less inter-bedded sandstone,-the latter predominating in Southwestern Colorado,-while the Permo-Carbon- iferous consists largely of variegated sandstones frequently conglom- eritic. Coal is rarely present in the true coal measures, having been observed at only three localities, viz .: Near Villa Grove, in the San Luis Valley, at Aspen just over the ore-zone, and near the head of the Huer- fano River. None of the coal beds are of workable size except the one
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HISTORY OF COLORADO.
near Villa Grove, and the coal is in each case of inferior quality. Probably the best defined exposure of the entire series of Carboniferous strata occurs on the. Rio Las Animas in La Plata County. The series is also well exposed at different points in Garfield, Pitkin and Eagle Counties, especially near Glenwood, and above Dotsero on Grand River, likewise in the Mosquito Range, and along the eastern flank of the Sangre de Cristo Range.
The life of the Carboniferous in Colorado, in common with that found elsewhere in the rocks of this age in the West, was throughout mainly marine; while in Eastern North America and in Europe the coal measures were mainly fresh water deposits, as shown by the numerous seams of coal, and by the remains of a luxuriant land vegetation.
Nowhere throughout the Rocky Mountains does it appear that the conditions necessary for the formation of coal (extensive swamps and exuberance of vegetable life) ever had more than a comparatively brief and extremely local existence. Sedimentation took place either in deep waters surrounding a precipitous coast, or along the shores of seas with strong currents; in the former case giving rise to calcareous deposits, and in the latter to sandstones and conglomerates. The Rio Las Animas strata are highly fossiliferous, especially below the mouth of Hermosa Creek, where crinoid stems, bryozoans, and characteristic Carboniferous marine shells are quite abundant. The remains of a few land plants, mostly ferns, are present in the exposures along the stage road running from Rockwood to Rico. Permo-Carboniferous shells are abundant at one point in the pinkish, or purplish, coarse sandstone exposed on the hillside a short distance west of Hermosa Creek. Marine fossils are likewise quite numerous in some of the Carboniferous strata above Dotsero on Grand River, especially near the mouth of Sweetwater Creek. Elsewhere in this series fossils are less abundant, though careful search will generally reveal them.
The great Palæozoic Era terminates with the Permo-Carboniferous, and the close of this period witnessed, everywhere, the extinction not
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HISTORY OF COLORADO.
only of all Paleozoic species, but of nearly all the genera. The Permian was the period of transition from the Paleozoic to the Meso- zoic,-from the ancient life era to the middle life era. In it the types of ancient life still predominated ; while as precursors of the coming life true reptiles made their appearance, amphibious having already appeared earlier in the Carboniferous.
Comparing the Rocky Mountain Palæozoic with the corresponding era in Eastern North America, these facts are noticeable,-that the amount of sedimentation was much less,-that the conditions for the existence of life were probably less favorable,-that during the coal period the topographical conditions were unsuited to the growth of extensive swamps or marshes, necessary for the formation of continuous beds of coal, and finally,-that the era was not brought to a close by grand dynamic manifestations such as marked the great Appalachian revolution. The transition from the Paleozoic to the Mesozoic in Col- orado took place without any serious break in the continuity of subsi- dence and sedimentation, so that the non-conformity between the rocks of the two ages is much less strongly marked than that already noted between the Carboniferous and the strata of Silurian or Cambrian Age.
MESOZOIC ERA.
This is the second grand division of time as applied to the develop- ment of terrestrial life, and the third in geological history. The three systems which it includes, the Triassic, the Jurassic, and the Creta- ceous, are all represented in Colorado, the latter especially, beside being the best defined and most extensively developed geological system in the State, is likewise economically considered the most important, for it was the great coal-forming period of Western North America; in this respect bearing the same relation to Rocky Mountain geology, that the Carboniferous does to the Appalachian.
The close of the Palæozoic witnessed a marked change in the geog- raphy of the continent. By the Appalachian revolution nearly all the country east of the Mississippi, to the Atlantic shore-line, was perma-
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HISTORY OF COLORADO.
nently elevated above the ocean level, forming an extensive land area in its main features corresponding to what we now find. In the country west of the Wahsatch, sedimentation continued on through the Triassic, when that portion of the continent also began to rise, and was probably dry land at the beginning of the Cretaceous. In the Rocky Mountain region, however, from Eastern Kansas to the Wahsatch Range, subsi- dence was more or less continuous throughout the entire Mesozoic, and the Colorado land areas were still but islands in the inter-continental sea. The dry land of the far western part of the State, remaining unsub- merged at the close of the Carboniferous, ceased to be such at the opening of the Mesozoic, for we find there the earliest sediments of this era reposing directly on the Archæan; indicating that this portion had not previously received sediments, and that it was formerly a Palæozoic island. The depression now represented by the basins of North and Middle Parks, which was probably a submerged area during the Palæ- zoic, although without any clearly established ocean connection, was undoubtedly submerged to a still greater extent during the Mesozoic, and formed a large salt-water bay directly connected by a narrow outlet with the main sea to the westward.
The two principal islands shown on the map as probably entirely separated during the early part of the Paleozoic were, as previously stated, no doubt permanently connected toward the north during the Carboniferous, and so remained throughout the Mesozoic. As all the systems of this era possess points of interest, it will be best to describe them separately, beginning with the beds of the oldest.
TRIASSIC PERIOD.
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