Geology and natural resources of Colorado

Front matter

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Table Of Contents

Chapter

PEE EIOT ee i BORNE lay Ce oh AM,

Rocks and Minerals, Topography and

Introduction

Geology is a history of the earth. The historian of a nation seeks the original documents, the ancient monuments, the institutions and even the traditions of the people whose history he would write. He finds in the past the key to the present, and in the present the natural consequence of the past. He brings together the materials of the past not as isolated, unrelated facts but as the warp and woof, the design, the coloring and the motive of a great tapestry, which becomes the living background and the stage itself on which is being enacted the drama of today.

The historian of the earth is equally dependent upon the past. He must seek the original documents—the rocks, their structures, the life records, the forces which produced the condition he finds. The records of the rocky pages are incontrovertible —unaffected, unaltered by national pride, hero-worship, hatred and jealousy. There is but one record—no conflicting transcriptions and translations, no conflicting motives of ancient chroniclers.

In earth history as in human history the light of the past fades into twilight and night, and the farther we seek the dawn the more impenetrable becomes the darkness.

In geological history there is a time when the light decreases with baffling suddenness, and though much the greater part of earth history preceded that time, the volume of our knowledge of that history is exceedingly limited, and the certainty of interpretation is small. We might call this time the dividing line between the unknown and the known—between the unknowable and the knowable, for there is but little hope of adding greatly to our knowledge of the almost limitless pre-Cambrian ages.

There is but one comprehensive geologic record—a very complex one. The great continental masses are the libraries, but so far as we. know not one of these contains the complete story, and it must be pieced together by careful study, comparison and correlation. If the continental records are incomplete, how much more so would be the story as told in the rocks of a single state such as Colorado.

The incompleteness of the record is due to the instability and changeableness of the earth. Even the mountains are but temporary features in the long earth history, and in the past many ranges have appeared and disappeared. Many subsidences and elevations of the land, many warpings and tiltings have caused countless changes in the relations of land and sea.

‘“There rolls the deep where grew the tree. O earth, what changes hast thou seen!

There where the long street roars, hath been The stillness of the central sea.

The hills are shadows, and they flow From form to form and nothing stands, They melt like mists, the solid lands, Like clouds they shape themselves and go.” —Tennyson.

These changes have controlled in large measure the writing of the most legible part of the earth story.

The greatest chronicler of earth history is water—chiefly marine or sea water, but land waters have played their part. The atmosphere, volcanic phenomena, climatic conditions and many other agents have added to the records but in a less important measure. And so the geological story of Colorado will be told largely in the changing relations of land and water. When water occupied part or all of the surface of the state, history was recorded in readily legible characters. When the area of the state was entirely land it was as though the pen had been lifted from the page, and we must seek the connecting links of the story, the lost act or scene of the play, elsewhere.

Just as human history is divided into eras, periods, epochs and ages by the rise and fall of races and peoples, so the local geological history of the earth is marked by events and changes which may serve to set off one part of geological time from another. As there have been very few events in human history which have involved the whole human race, so there have been few geological events which have affected the whole earth, or even a whole continent. One region may pass through a period of upheaval and mountain building while another part of the same continent xi may be undergoing a depression which allows the sea to cover vast areas once occupied by dry land. While volcanoes may be burying forests and fields under molten lava in one part of a continent, glacial ice may be accumulating in another region at no great distance.

Students of geology, the world over, have accepted the main subdivisions of geological time adopted by the pioneers of the science in Europe. Later study has shown that these subdivisions are not readily recognized in all parts of Europe and have but a limited application to the geological history of America. At the same time, though these divisions of geological time are, for many parts of the earth, purely artificial, probably no other dividing lines could have been chosen which would have been based on more widespread geological events. They are universally adopted because they serve the useful end of grouping life forms and geological events for the purposes of world wide study and comparison.

The history of life on the earth is the story of the development of plants and animals from simpler to more complex forms. It is true that some ancient types still live, connecting the remote past with the present, but if we could compare the life of the earth today with that of early geological time, the differences would impress us much more strongly than would the likenesses. And yet if we could have before us a grand procession representing the life of all geological ages from the dawn of life on the earth to the present time we should be impressed by the gradual development from the earlier and more primitive forms to those of the present time. We should find no great breaks or abrupt changes marking the passage from one geological era to another—no sudden appearance of new and unexpected forms—but a gradual unfolding and upbuilding. This is very well illustrated by the evolution of the horse, whose development from ananimal not larger than a house cat to his present size occupied millions of years. But in all that time there were no abrupt changes, but rather a slow, steady progress toward the present form.

The history of life on the earth might be compared to the current of a river—unbroken from the mountain springs to the ocean. The rate of flow varies from place to place through pools and eddies in which motion is scarcely measurable, then past narrows and over rapids where it is strong and swift. So the rate of life development was at times sluggish, at times rapid, dependxlil ing upon climate, food supply, density of population and other local conditions.

The divison into eras is based on the broad, general features of the development of life as revealed by the study of fossils and other evidences of life contained in the rocks. The divison of the eras into periods does not follow any rule. Several periods are named from the regions in which the rock systems of those periods were first studied, or most satisfactorily interpreted. The name Cambrian is from “‘Cambria,’’ the Latin name for Wales. Ordovician and Silurian come from “Ordovices’’ and ‘‘Silures’’, the names of two ancient British tribes. Devonian is from the county of Devon in southwestern England. The Carboniferous period is so named on account of the great amount of carbon (in the form of coal), contained in the rocks of that period. The Triassic is named from the threefold division of the rocks of the period in Germany. The Jura Mountains furnish us the name Jurassic. The Cretaceous is so named from the abundance of chalk (creta) in the rocks of that period, in England.

The larger subdivisions of geological time, as used in America, are shown in the following table.

X1V

Geological Time Table Era Period

Recent Quaternary.{ sent

Pleistocene en OZOIG bo i ut Pliocene (New Life) | Tertiary _.... Miocene Oligocene — Eocene

(Middle Life) | Jurassic

| Triassic

(Permian | Pennsylvanian (Upper Carboniferous) Mississippian FLCC LONG ee GU (Lower Carboniferous) (Old Life) Devonian Silurian Ordovician Cambrian

Great Interval hatinaing (Interval) | Upper Huronian

(Interval) BARE TOLETOZOIC Ui eM ar uy La LU, Middle Huronian ‘| (Dawning Life) (Interval) E Algonkian ae Huronian O (Great Interval) 2 ” | {Keewatin | Laurentian PATCheazoig wan ul ve The great Archean complex (Ancient or First Life) of granites, gneisses, | Archean | schists, slates, quartzites, letc.

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CHAPTER |

ROCKS AND MINERALS, TOPOGRAPHY AND DRAINAGE

ROCK TYPES

There are three general rock types recognized by geologists:

|. Igneous rocks are those which have solidified from a molten condition.

2. Sedimentary rocks are those derived by processess of decay and wear of pre-existing rocks. The materials resulting from such processes are carried from their place of origin and deposited elsewhere chiefly by water and in water, though wind may carry and deposit the finer and lighter materials, such as sand and dust.

3. Metamorphic rocks are those which have been formed by the profound alteration of igneous or sedimentary rocks. The chief agencies producing such alteration are heat, water, pressure and movement.

Most rocks are composed of minerals, and though there are thousands of minerals known to geologists only a few are important in the makeup of rocks. The three rock types have their characteristic minerals, but some are found in all types.

If molten rock matter cools and solidifies very slowly it is commonly completely differentiated and crystallized into mineral grains, and the texture is fairly coarse. If it cools more rapidly it may be wholly differentiated and crystallized into mineral grains, but they will be small and the rock is fine textured. Rapid cooling will form a rock which may be only partly crystalline and partly glassy, or wholly glassy.

. The lavas poured out of volcanoes or fissures are fine textured and often glassy because of rapid cooling. Granites are coarse textured because they cooled slowly under the protecting cover of other rock masses.

The commonest rock-making minerals of the igneous rocks are: Feldspar, quartz, mica, hornblende, pyroxene, olivine and the oxides of iron.

GEoLocY AND NaTuRAL REsourRCcES OF COLORADO

The minerals of the sedimentary rocks are:

Quartz in the form of sand.

Feldspar in fragments and sand.

Calcite or lime carbonate.

Gypsum or lime sulphate.

Kaolin which is an important constituent of clay rocks. Magnetite is present in most sedimentary rocks.

Feldspar is a silicate of aluminum with potash, soda or lime. It is commonly pink or milk white and is by far the most important mineral of igneous rocks.

Quartz as a rock-making mineral is generally in irregular grains, is very hard and has a glassy luster.

Mica is readily recognized by its perfect cleavage into thin, smooth, shiny flakes. There are two common micas. Muscovite is white or silvery. Biotite is black.

Hornblende and pyroxene are closely related minerals which cannot be readily distinguished in the rocks. The commoner forms of these minerals are greenish gray to iron gray or black. They do not glisten as does the black mica and are usually in longer grains of fibrous appearance.

There are pale green and grayish green fibrous hornblendes and fibrous bronzy pyroxenes.

Olivine is a rather rare glassy mineral of yellowish green color.

The commonest iron minerals are the heavy black magnetite, and the brass-yellow pyrite.

Calcite is a colorless to white mineral with a good cleavage. It is the granular mineral of marble. It is soft and easily scratched.

Gypsum is a white granular mineral, soft and easily cut with a knife.

Garnet is a common mineral in metamorphic rocks. It is very hard, resists weathering, and is generally easily recognized by its garnet red color and glassy luster.

The micas, hornblendes and quartz are important minerals in the schists and gneisses.

Rocxs anp MINERALS, TOPOGRAPHY AND DRAINAGE 3

OCCURRENCE OF IGNEOUS ROCKS

Igneous rocks are grouped, according to their mode of occurrence, into [ntrusive and Extrusive rocks. Intrusive rocks are those formed by the solidification of molten rock matter which has risen to a point near the surface, but has cooled under a cover of rocks. Extrusive rocks are those formed by the solidification of rock matter which has been forced out upon the surface of the earth through volcanic or other vents.

Intrusive: Bodies of intrusive rock are classified according to their form as: dikes, sheets, laccoliths, batholiths, necks or plugs, and stocks or bosses.

DrxEs are the result of the solidification of molten rock in fissures. They are, as a rule, many times as long as wide (or thick), and they commonly reach the surface in a direction approaching the vertical.

SHEETS are masses of rock which in their ascent toward the surface have spread out between the strata of a sedimentary series. Near Golden sheets have been formed by the burial of surface flows (extrusive) beneath sedimentary beds.

Laccotitus: When a sheet is so thickened as to form a lenticular mass arching the overlying strata, it becomes a laccolith.

BaTHOLITHS are great, irregular masses, often hundreds or thousands of square miles in area. They are mostly associated with the very ancient rocks, and are frequently in contact with schists and gneisses. Batholithic outcrops are numerous in a zone extending from Boulder County southwest to the San Juan.

Necks AND Puucsare roughly cylindrical or spine-like columns of volcanic rock projecting from what are believed to be the conduits or throats of extinct volcanoes. They are supposed to result from the solidification of the magma which filled the conduit, and their projection above the surface is due to their greater resistance to erosion than the volcanic cone or the rock through which they project. Examples are to be seen in the Bear River valley near Yampa.

Stocks or Bosses are rounded masses of igneous rock somewhat resembling necks, but usually of greater size. The openings through which the rock rose to its present position may never have reached the surface. Several stocks occur in the San Juan region.

4 GEOLOGY AND NaturAL RESOURCES OF COLORADO

Extrusive: Extrusions of rock occur in three general ways: (a) wellings from fissures; (b) welling volcanic eruptions of molten rock, sometimes more or less mingled with fragments of solid rock; (c) explosive volcanic eruptions. In several parts of the earth large areas have been covered by sheets of lava which has welled up through fissures and flowed out over the surface of the earth. In some instances numerous sheets have succeeded one another and resemble the strata of a sedimentary series. The Snake and Columbia River areas and parts of the San Juan area furnish good examples of this mode of occurrence. Potosi Peak is carved out of a series of lava flows. Sheets of this kind are likely to be of fairly uniform thicknessand of wide extent.

The eruptions of many volcanoes consist mainly of quiet up-wellings of lava, until the craters are filled and the molten rock flows down the slopes, or until the pressure of the lava upon the crater walls breaks them and the lava flows out through the gap. Sheets or flows formed in this way are likely to be of irregular thickness, stream-like in cross-section, and of no great lateral extent.

Some volcanoes erupt with explosive violence and send cut vast volumes of solidified rock matter in fragmental form. Such material is classified, according to its degree of fineness, into dust, ash, lapilli, bombs, blocks. The finer material, when consolidated, is called tuff; the coarser forms, breccia and agglomerate.

CLASSIFICATION OF IGNEous Rocxs

(See Illustrations Pages 5’ to 25)

I. Granitoip Rocks: All rocks whose mineral grains are large enough to be recognized by the unaided eye.

I]. Frrstrorp Rocxs: Rocks composed mainly or entirely of mineral grains so small that they cannct be recegnized without the aid of a strong lens. Here belong also porphyritic rocks having a felsitoid groundmass.

III. Gztassy Rocxs: Those composed so largely of glass that they have a vitreous or glassy luster.

IV. Fracmentat Rocxs: Including chiefly tuffs and breccias. Rocks composed mainly of vclcanic detritus, ranging in texture from dust grains to fragments several inches in diameter.

Rocxs AND MINERALS, TOPOGRAPHY AND DRAINAGE

Dikes Spanish Peaks, Colo.

Basic dikes in light colored quartzite

Volcanic neck or plug Yampa Valley, Colo.

Rocks AND MINERALS, TOPOGRAPHY AND DRAINAGE 7

==

Section through a laccolith before erosion Hahns Peak ica Zl oe wen

The iw y

Section through an eroded laccolith Hahn’s Peak, Colo.

Final wes be eens

Series of lava flows

8 Gro_ocy AND NaturaL REsouRCES OF COLORADO

Granitoid, fine texture

Granitoid, coarse texture Granitoid, uneven texture

GRANITE GROUP

Rocks anp MINERALS, TopoGRAPHY AND DRAINAGE 9

Quartz-feldspar porphyry Syenite porphyry GRANITE GROUP

10 GEOLOGY AND NaTuRAL RESOURCES OF COLORADO

Glassy and porphyritic texture Felsitic . texture pitchstone porphyry felsite Georgetown, Colo. Boulder, Colo. g

GRANITE GROUP

Rocxs anp MINERALS, TOPOGRAPHY AND DRAINAGE 11

Pumice

Rhyolite tuff Rhyolite breccia

Syenite

Monzonite Quartz monzonite

Silverton, Colo. Chaffee Co., Colo.

Rocxs AND MINERALS, TOPOGRAPHY AND DRAINAGE 13

Trachyte Trachyte showing flow structure sot alone ® % Hornblende latite Quartz latite Silverton, Colo. Silverton, Colo.

14 GEoLocy AND NatrurAL REsourRCES OF COLORADO

Nephelite syenite Cripple Creek, Colo.

Phonolite Vesicular phonolite

Cripple Creek, Colo. Cripple Creek, Colo.

Rocks anp MINERALS, TOPOGRAPHY AND DRAINAGE 15

Diorite oe

Andesite Porphyritic andesite

16 GroLtocy anp Natura RESouRCES OF COLORADO

Andesite breccia Andesite tuff

Cripple Creek, Colo.

Hornblende gabbro

Rocxs AND MINERALS, TOPOGRAPHY AND DRAINAGE i pace Valmont, Colo.

Pyroxenite Peridotite

18 GEOLOGY AND NaTuRAL REsouRCES OF COLORADO

SUBDIVISION OF THE Groups

I. Granitoid Rocks

1. Granite: Chief minerals are Feldspar and Quartz. Subordinate minerals may include mica, hornblende, pyroxene. Granite porphyry may be included.

2. SyenitE-MonzoniTeE: Chief mineral is Feldspar. Subordinate minerals may include quartz, mica, hornblende, pyroxene and nephelite.

3. DzoriteE: Chief mineral is Hornblende. Subordinate minerals are mica, feldspar, pyroxene.

4. Gassro. Chief mineral is Pyroxene. Subordinate minerals may be feldspar, hornblende, olivine and mica.

5. PeripotiTe: Olivine is very abundant. Subordinate minerals may include pyroxene, hornblende, feldspar and mica.

6. PyroxeNiTE: Pyroxene makes up almost entire rock.

7. HorNBLENDITE: Rock is almost entirely Hornblende.

II. Felsitoid Rocks

|. Fexsrre: All light-colored, felsitoid rocks, with few or no phenocrysts. The colors may include white, gray, buff, yellowish-brown, pink and greenish. The group will include most of the rhyolites, many of the andesites, most of the trachytes, latites and phonolites.

2. FevsitrE-Porpuyyry: All light-colored porphyritic felsitoid rocks, no matter what the phenocrysts may be. The group will include most of the porphyritic members of the felsitoid groups.

3. Basatt: All dark-colored felsitoid rocks with few or no phenocrysts. The colors may be dark gray, brown, dark green, black.

4. Basatt-Porpoyry: All dark-colored porphyritic rocks with felsitoid groundmass.

III. Glassy Rocks

Those composed of glass or having a glassy Luster

1. Ossip1an: Non-porphyritc, glassy or vitreous rocks of any color. The common colors are black, red, brown and greenish.

RocKxs AND MINERALS, TOPOGRAPHY AND DRAINAGE 19

2. Ossipran-Porpuyry: Porphyritic, glassy rocks of any color.

3. Pircustone: Non-porphyritic rocks of any color, having a resinous or sub-vitreous luster.

4, Pitrcustone Porpuyry: Porphyritic rocks having a resinous or sub-vitreous luster.

5. Periire: Glassy rocks with perlitic structure.

6. Pumice: Highly vesicular rock glass, usually very light-colored.

IV. Tuffs and Breccias (Fragmental)

These may be divided into felsite tuff and felsite breccia (light-colored), and basalt tuff, basalt breccia (dark-colored).

SEDIMENTARY Rocks

In this group are included all rocks derived from pre-existing rocks by processes of weathering or decay, by chemical change, chemical precipitation or life processes. They are grouped as follows:

1. Sand and gravel rocks.

2. Clay rocks.

3. Rocks chiefly of organic origin.

a Lime rocks, such as limestones.

b Carbonaceous rocks, such as coals.

c siliceous, such as geyserite.

d Phosphatic—rock phosphate. _

4. Chemical precipitates, such as rock salt, gypsum and others.

1. Sand and gravel rocks include loose sand, gravel, boulders, and the same materials cemented into sandstone, pebble rock, conglomerate. The bonding materials may be lime carbonate, silica cr quartz, clay or iron oxides. |

2. Clay rocks include mud, soil, loam, shale, clay shale. Sand, lime carbonate and other substances may be present, and there may be transitions from clay rocks to limestones and sandstones.

3. Rocks of organic origin include those in the making of which life processes have played a part.

20 GEoLocy AND NatrurAL RESOURCES OF COLORADO a lLimestones are made of lime and carbon dioxide. The lime comes primarily from the igneous rocks and the carbon dioxide from the air or from the interior of the earth. In the weathering of rocks these two unite and form lime carbonate which finds its way to the sea, where it is used by the sea animals to form their shells and other hard parts. At the death of the animals these hard parts accumulate on the sea floor and in time are converted into firm limestone rock. Limestones are rarely pure, but generally contain some clay, silica, iron and magnesia.

b Carbonaceous rocks include coals, asphalts, bitumens, oil shales and oil. The carbon of these is derived mainly from the air by plants and animals.

c Siliceous rocks, such as diatomaceous earth, consist of silica secreted by life agencies.

d Phosphate rock is a phosphorous-bearing rock formed from animal remains, chiefly calcium phosphate.

4, Chemical precipitates, such as rock salt, gypsum and others are precipitated from sea waters in shallow bays and enclosed lagoons by the evaporation of the sea water.

MetTAmMorRPHIC Rocxs

The common metamorphic rocks are: Gneiss (pronounced nice), schist, slate, quartzite, marble.

GNEIss is a rock in which the component minerals are more or less segregated into irregular alternating layers giving it a streaked appearance. —The commoner gneisses are derived from the various granitoid igneous rocks and are called: Granite gneiss, syenite gneiss, monzonite gneiss, and so on. If the rock shows but little of the segregation it may be called a gneissoid granite, gneissoid syenite and so on.

Scuist is a rock in which a fairly perfect segregation of the component minerals has taken place, causing the rock to have a thinly laminated structure along which it splits easily. The

Rocxs

AND MINERALS, IOPOGRAPHY AND DRAINAGE

Alternation of hard and soft strata (limestone and shale)

Conglomerate

Boulder, Colo.

Sandstone

Boulder, Colo.

Banded sandstone

Rocks AND MINERALS, TOPOGRAPHY AND DRAINAGE 23 commoner schists are: Mica schist in which one of the micas is very prominent. The other minerals may include quartz, hornblende, some feldspar. Hornblende schist in which hornblende is abundant. Quartz is the most common other mineral. Schists are formed from both igneous and sedimentary rocks by metamorphism.

SLATE is derived from clay rocks by a process of lamination and partial or complete crystallization. The minerals resulting from crystallization may include mica, pyrite, quartz, calcite and others. The softer slates are called clay slates.

QUARTZITE may be defined as a sandstone which has been so changed by metamorphism that it no longer breaks around the sandgrains but through them. The bonding of the grains is as strong as the grains themselves.

MARBLE is a metamorphosed limestone. In the change it has become crystallized, commonly whitened, and sometimes mottled. Impure limestones make colored marbles.

TOPOGRAPHY

Colorado is divided into three topographic zones—the eastern plains, rising from an elevation of a little less than 4,000 feet along the eastern border to 6,500 feet at the mountain front; the Rocky Mountain zone, the east side of which rises abruptly from 6,500 feet to the crests of the ranges where many peaks are over 14,000 feet high; and the western plateau zone, extending to the Utah line.

About two-fifths of the state lies in the eastern plains zone, and the remainder is about equally divided between the mountain and plateau zones. The prominent features of the eastern zone are the shallow valleys of the Arkansas and South Flatte rivers, and the broad flat divide between them.

The northern boundary of the state in this eastern zone follows closely the flat divide between the North and South Flatte rivers. The head waters of the tributaries of the South Flatte have cut narrow valleys into the divide.

The divide between the Arkansas and the Cimarron rivers is plateau-like and is cut into deep canyons by the tributaries of the two rivers. At the eastern border of the state the divide is about half way between the Arkansas and the New Mexico line. Farther

Biotite schist

Rocks AND Mrnerats, TopoGRAPHY AND DRAINAGE 25

Mottled marble Onyx marble

26 GEo.Locy AND Naturat REsourcES OF COLORADO west it approaches the New Mexico line and crosses it near the west end of Mesa de Maya.

The Rocky Mountain zone is a complex of ranges. Facing the eastern plains from north to south are: ‘The Front Range extending from beyond the Wyoming line to Pikes Peak, and the Sangre de Cristo from the Arkansas River to the New Mexico line. The Wet Mountains are an isolated range parallel to and east of the Sangre de Cristo, extending from the Arkansas to the Huerfano.

Various local names are given to divisions of the Front Range, such as: Medicine Bow, a northern spur between the Laramie and the North Platte rivers; the Snowy range between Clear Creek and Grand counties; the Rampart Range from South Platte River to Fountain Creek; Tarryall Mountains; Kencsha Hills, and others. The Culebra Range is that part of the Sangre de Cristo immediately north and south of the New Mexico line.

The Park Range extends from the Arkansas River in Fremont County northward through Colorado into Wyoming. Such local names as Mosquito, Gore, and others are applied to parts of the Park Range.

Between the Front Range and the Gore and Park ranges are minor connecting ranges and spurs such as the Rabbit Ears, Vasquez, Williams River and others. The Sawatch range on the west side of the Eagle and Arkansas valleys is one of the mast massive in the system. In it are Mount Elbert and Mount Massive, the two highest peaks of the state, the Mount of the Holy Cross, and the Collegiate Group, all over 14,000 feet in height. West of the Park Range lie the Elk Head Mountains in Routt County, and west of the Sawatch are the Elk Mountains in Gunnison County.

The San Juan Mountains are a massive, rugged group in the southwest corner of the state. Many peaks are over 14,000 feet high. The Needle Mountains of the San Juan contain the most precipitous peaks in the state.

Between the Front and Park ranges are the North, Middle and South parks. The North and the South parks are great level expanses ranging in elevation from 8,000 to 10,000 feet. Middle Park consists of a series of minor valleys separated by mountain ranges of considerable height.

RELIEF MODEL OF COLORADO a

Rocks AND MINERALS, TOPOGRAPHY AND DRAINAGE 27

Between the Sangre de Cristo and the San Juan Mountains lies the great level floor-like San Luis valley at an altitude of 7,000 to 8,000 feet.

The east end of the great Uinta Arch extends into the northwestern corner of Colorado from Utah.

The plateau zone consists of a great series of mesas or plateaus flanking the mountains and declining gradually or by step-like intervals to the western borders of the state. Their surfaces are fairly uniform. Some are horizontal, others are inclined at various angles toward the west, northwest and southwest. The valleys about their borders are usually canyons.

DRAINAGE

The continental divide forms a very irregular line lying mainly between Meridians 106° and 107° west. It parts the waters of the state, sending those of the east slope to the Gulf of Mexico by way of the Mississippi and the Rio Grande, and those of the west slope to the Gulf of California by the Colorado. The principal rivers of the east are: The North Platte, South Platte, Arkansas and Rio Grande, all of which rise in the mountains and pour their waters through deep and rugged channels to the plains, where they are fed very meagerly by the various plains tributaries, most of which are intermittent. As the precipitation in the Rocky Mountain zone is heavy, the volume of water originating in the state is comparatively large.

The rivers of the west slope are: The Colorado and its tributaries: the Bear (Yampa), White, Gunnison, Dolores,and San Juan, all of which head in the mountains, where they receive numerous tributaries, and flow to the more arid plateau country, receiving small contributions from the intermittent streams from the plateaus.

The rainfall ranges from 40 inches per year on some of the divides to 10 inches per year in parts of the western plateau zone. The precipitation is largely confined to the six months from May to October, and, as a consequence, the stream flow is subject to notable seasonal variations.

Within the mountain and plateau zones the canyon is the typical form of valley. On the eastern plains this type is not