Central City and Idaho Springs

The Central City-Idaho Springs area was the principal metal mining region in the state until the late 1880s. In 1858, richplacerdeposits were discovered in gravels and river terraces along both forks of Clear Creek. Exploration upstream led to discoveries of rich oxidized quartzveinsat Central City, Black Hawk, and Idaho Springs. These veins, which generally trend northeast-southwest, extend through the mountains in a zone about six miles long and three miles wide between the two forks of Clear Creek.

The ores filled a multitude of cracks and fissures in the Precambrianbedrock. Theveinsare usually less than five feet thick, and are almost vertical and often clustered in zones up to thirty feet wide. The position of one of the vein systems may be seen clearly between Black Hawk and Central City—the ore-bearing rock has been mined out, but a series of collapsed tunnels marks the line where the veins crossed the valley. A monument here commemorates the discovery of Gregory Gulch, one of the richest localities in the state.

Several richveinswere mined in both directions—southwest from Central City and northeast from Idaho Springs—until the mines met. The Argo tunnel, marked by dilapidated buildings and extensive dumps on the north side of Idaho Springs, connected the two districts; it was completed in 1904.

The “Patch,” a deep crater-like hole on Quartz Hill, about one mile southwest of Central City, is an intriguing feature in this area. It was produced by glory-holing, a mining technique in which a deep tunnel is deliberately caved by blasting, so that ores above the tunnel can be removed. This glory hole was dynamited below an irregular mass of highly broken rock where many ore-richveinsconverged. After the caving, ores were taken out through the remaining part of the tunnel.

The principal ore minerals of Central City and Idaho Springs arenative gold,pyrite,sphalerite,galena,chalcopyrite, andtennantite. Prospecting for uranium was carried out during the 1950s but no uranium was ever mined here.

The area has produced almost $200,000,000 worth of gold, silver, lead, zinc, and copper. A few mines still operate seasonally or on a small scale, but tourists, many of them riding Jeeps across the mountainous terrain to visit mines and ghost towns, are often more visibly active than the mines.

A few miles southwest of Idaho Springs, another mining area had a similar, though less productive, history. In 1859,placerandlodegold were discovered near what is now Georgetown. Placer mining dominated here between 1859 and 1863. Gravel and crushed rock from decomposed quartz and sulfideveinswere washed through sluiceboxes in the same way as placer gravel, gold being caught in riffles or gunny sacking on the bottoms of the troughs. The veins were found to be decomposed to depths of about 40 feet; below this the gold occurred closely associated with sulfides such aspyrite,sphalerite,galena, andchalcopyrite, from which it could not easily be separated. However, smelters were developed in 1866 for treatment of these sulfides, and gold, silver, lead, and copper were recovered. Gradually, as the gold was worked out, silver and lead became the important products of the mines.

Sluicebox mining was a common sight near the early gold camps, where primary recovery was fromplacerdeposits or decomposed quartz and sulfideveins. (State Historical Society of Colorado photo)

Placergold was discovered in 1859 in California Gulch, about seven miles north of the present town of Leadville. The rush that followed was short but sweet; the camp was called Oro—gold! About $5,000,000 was produced from the placer mines within two years, though by 1861 the area was all but deserted, for the easily won placer gold was gone.

Early-day Leadville sprawled among its mine dumps at an elevation of 10,200 feet. The Sawatch Range, in the background, contained many smaller mining communities, now deserted. Mt. Massive, the state’s second highest peak, forms the crest of the continental divide here. (State Historical Society of Colorado photo)

In 1875 a smelter was erected a few miles downstream from Oro to process cerussite—silver-rich lead carbonate—that occurred in theplacersands. For years this mineral had been considered a nuisance because, being much heavier than sand, it tended to separate out with the gold. The new town of Leadville sprang up near the smelter and shortly afterward morelodedeposits were discovered south of the placer workings. From $63,000 in1875, production climbed to $2,500,000 in 1878 and more than $15,000,000 in the peak year of 1882.

Geologically, the ores of this district occur as Tertiary replacements andveinsin Ordovician, Devonian, and Mississippian limestones. The “Blue” or Leadville Limestone, of Mississippian age, contains the richest ore. Ore deposits were formed after the limestones had been faulted and cracked extensively by mountain-building movements; the ores themselves probably crystallized from molten or gaseous materials involved in related igneous intrusions. River gravels and glacial debris mask the true nature of thelodedeposits, but studies in the mines show that thefaultsystems along which ores are deposited trend north or north-northeast.

The Leadville district is now experiencing its third mining boom as a newly recognized lead-zinc orebody is being developed. Production is expected to reach 700 tons of ore per day by 1971. Total production of gold, silver, lead, zinc, and copper in the district has reached $500,000,000.

Breckenridge was also discovered in 1859, withplacergold the first attraction. The placers gave out in 1862 after about $3,000,000 in gold had been recovered. Earliest attempts to mine the rich silver and leadveinsof the district were in 1869.

As at Leadville, thesedimentary rocksof the area were intruded by granitic masses in Tertiary time, but here the sedimentary rocks are mostly Pennsylvanian sandstones and shales. These rocks were badly faulted and broken during the intrusion, and the ores were deposited as the granitic material cooled. Thelodedeposits occur mostly in smallveinswell hidden by surface sands and gravels. Some of the veins yielded exceptionally beautiful crystallized wire and flake gold, specimens of which are on display at the Colorado School of Mines library in Golden and in the Denver Museum of Natural History.

Dredging for alluvial gold was attempted in 1898 in the Breckenridge district, but this method of extracting gold was not successful until 1905. A number of dredges operated between 1910 and 1925. These floating behemoths shovel up gold-bearing gravels from the bottom and one side of the pond on which theyfloat, sort out the gold in giant sluiceboxes, and spew out the leftover gravels in great arc-shaped heaps that can be seen near Breckenridge and Fairplay and in a number of other valleys in Colorado. They depend for their operation on a plentiful supply of water and a shallow water table, but they can sift through quantities of gravel at relatively low cost. All told, about $7,000,000 in gold has been dredged from this district.

This gold dredge, still floating in its pond just south of Fairplay, operated from 1941 to 1952. With chains of buckets like those in the foreground, it dug gravel 70 feet below water level, carving a 35-foot bank above water level; in effect it mined to a depth of 105 feet. This dredge extracted nearly 115,000 ounces of gold from about 33 million cubic yards of gravel (John Chronic photo)

Another gold field discovered in 1859 was in the northwest corner of South Park, along the headwaters of the South Platte River. Several mining camps were established here. After early production of richplacerdeposits, claims were consolidated and large flumes constructed so that gold could be recovered byhydraulic mining. In this type of mining, streams of water from high-pressure hoses are directed at gravel surfaces. The gravels are washed into long sluiceboxes, where gold is caught in riffles. Hydraulic mining continued upstream from Fairplay until about 1900.

In 1922 a dredge was constructed near Fairplay to process gravel along the South Platte and in the valley floor. An even larger dredge, constructed in 1941, operated until 1952, when rising labor costs overrode the narrow margin on which it operated. At the time operations ceased, the dredge was recovering about six cents in gold for each cubic yard of gravel processed.

Placergold has always been the principal mineral product of the Fairplay area, butnative goldalso occurs in the surrounding mountains in quartzveins, and many small mines were developed to extract it. Sulfide ores were also mined; they contained silver, lead, and zinc as well as gold. In the Mosquito Pass and Horseshoe Amphitheater areas, there is renewed activity now because of the recent rise in the price of silver.

Gold was discovered in the San Juan Mountains of southwest Colorado in 1870. The earliest mine, near what is now Silverton, was located by a group of prospectors sent out by Governor Pile of New Mexico Territory. Since the site was on Ute Indian land, real mining did not begin until a treaty allowing it was ratified in 1874.

Production in the Silverton district has been fromveinsin Tertiary volcanic rocks within an elliptical area known as the Silverton cauldron. Here the volcanic rocks, part of the several thousand feet oflavaflows and ash falls of the San Juan volcanic field, were cracked and faulted by a second period of igneous activity. Ores formed in the cracks and fissures.

In the 1870s the Silverton district was very remote, and difficulties with transportation retarded activity there. In 1882, however, a narrow-gauge railroad was built connecting Silverton with Durango, and the problem of transporting ore out of the isolated mountain valley was simplified. The railway still exists; a train makes daily passenger runs during the summer—the only remaining operating narrow-gauge line in the United States. Thetrack follows the Animas River canyon, whose cliffs and crags are dotted with long-abandoned mines, prospect holes, and mine buildings, monuments to the tenacity and determination of the men who mined here.

Production in this district was more than $22,000,000 in gold and $20,000,000 in silver between 1874 and 1923. New activity is evident here, as in other silver-rich areas of Colorado, because of recent demand for silver, lead, and zinc.

Silverton lies in a remote mountain valley in the San Juan Mountains. Silver, gold, lead, and zinc have been mined here since 1874. Storm Peak, composed of Tertiary volcanic rocks, forms the backdrop; the narrow-gauge railroad track is visible in the foreground. (Jack Rathbone photo)

Ouray was settled in 1875, when gold and silver deposits were found near Mount Sneffels. Since 1877, mines in Ouray County have produced over $35,000,000 in gold and $32,000,000 in silver. The district is still quite active: in 1965, mines in this area produced more than $9,000,000 in gold, silver, copper, lead, and zinc, about a third of total Colorado production of these metals for that year.

A few miles south of Ouray, along Uncompahgre Gorge, an old mine clings to the slope below the Million Dollar Highway (U. S. 550). Abrams Mountain rises in the background. The Precambrian Uncompahgre Quartzite outcrops up to about the road level; Miocene Sunshine PeakRhyolitecaps the peak. (Jack Rathbone photo)

A mile north of Ouray a prominent intrusivestockmarks the center of mining activity closest to Ouray. The richest deposits of the Ouray area, however, lie about five miles southwest, nearMount Sneffels and Red Mountain Creek. There, several large mines, including the famous Camp Bird mine, have operated for many years, extracting ore from hundreds ofveinsthat underly the surface. Some of these veins are two to four miles long. They are in Tertiary volcanic rocks of the San Juan Formation. Quartz and calcite are the commongangue(non-economic) minerals, andpyrite,sphalerite,galena, andchalcopyriteare the most abundant ores. Most of the silver is in the galena; gold occurs in streaks and nodules associated with quartz.

About ten miles south of Ouray, along the “Million Dollar Highway” (U. S. 550), the Red Mountain district lies on the northwest edge of the Silverton volcanic cauldron. It contains a number of small pipelike bodies very rich in silver-copper and silver-lead ores. Following the mid-Tertiary volcanism and ore intrusion, surface rocks in this area were intensely oxidized: resulting iron oxides now form the gaudy reds and yellows of Red Mountain and the slopes near Ironton. This alteration, as well as the fact that much of the area is covered with fallen rock, stream gravels, or glacial deposits, compounds difficulties of locating the small though high-grade ore deposits.

The Idarado Mine, on the east side of U. S. highway 550 near Red Mountain, used to produce ores from nearby volcanic pipes; now it produces fromveinssome distance to the northwest. The area is honeycombed with tunnels and shafts.

Silver was found at Castle Creek and on Aspen Mountain in 1879. A group of prospectors from Leadville, apparently after examining maps of the Geological and Geographical Atlas of Colorado published in 1877, explored along the line of Paleozoic limestones encircling the Sawatch Range. As they had hoped, they found ores similar to those at Leadville in rocks of the same age.

Mining began at Aspen in 1880. Here, as at Leadville, intrusion ofgraniteporphyryinto or near the Leadville Limestone had broken and deformed the layers, and ores were deposited in fissures and as replacements during cooling of the intrusions. The intricacy of faulting which controls the ore pockets in the limestone is well shown on the map ofAspen Mountainin Chapter II.

Glaciationoccurred in this area, and glacial deposits cover most of the ore bodies and outcrops so that littlebedrockisexposed. Mapping was accomplished by extrapolating to the surface the bedrock patterns shown in mine pits, shafts, and tunnels.

Aspen produced some of the richest silver ores in the world, and thrived as a boom town for most of two decades. In 1888 the value of ores produced reached over $7,000,000; the next year it topped $10,000,000. After the silver crash of 1893 production declined rapidly; the last mines were closed in the 1920s. Total production of silver, lead, zinc, and copper reached about $100,000,000. There was virtually no gold in the ores at Aspen.

Creede and its mines are located in an area of Tertiaryrhyoliteand dacite, light-colored volcanic rocks.

The Creede district ranks as one of the most productive silver areas in the United States. It came into being largely as a result of a discovery by N.H. Creede in 1889. When exploring in thisarea, he was reported to have exclaimed “Holy Moses!” on examining a rich piece of ore, thus giving the name to the mine which initiated the rapid development of the district. By the end of 1892 the Holy Moses and nearby mines had produced ore valued at more than $4,000,000. The area was so rich that it managed to survive 1893’s great decline in the price of silver; by 1920 almost $42,000,000 in gold, silver, lead, and zinc had been mined there.

The ores, silver-bearinggalena,sphalerite,native gold,pyrite, andchalcopyrite, are in quartz or amethystveinsin faulted and shattered Tertiary volcanic rocks. Nearly all the ore deposits lie along a complex system of verticalfaults, the Amethyst fault zone, which runs more or less northwest-southeast through this region. Both the faulting and the enrichment of the fault fissures are believed to have taken place in mid-Tertiary time, shortly after deposition of the volcanic host rocks.

Cripple Creek, on the flanks of the Pikes Peakmassif, has produced more than $400,000,000 worth of gold. The Sangre de Cristo Mountains are visible in the distance beyond the Arkansas River valley. (Jack Rathbone photo)

In 1890, two sheepherders stumbled on some richly mineralized rocks near Cripple Creek. A boom developed immediately, for the rocks contained both gold and silver. Since then, the areahas produced more than 2,000,000 ounces of silver and nearly 19,000,000 ounces of gold.

Cripple Creek has produced almost half of all the state’s gold and silver. The ores are located in or at the edge of a large mass of middle Tertiary volcanic rocks which form an elliptical basin orcalderaseveral miles across. The caldera, surrounded by Precambriangneissandgraniteof the Pikes Peakmassif, was probably formed by collapse of a volcanic center that had erupted through the older rock. The collapse shattered the rocks around the basin margin, and subsequent volcanic activity introduced mineral-rich solutions into the manyfaultsand fissures produced by the collapse. Tellurides of gold, silver, and copper, as well aspyrite,sphalerite,galena,tetrahedrite, and other minerals, are characteristic.

At Climax, the ore occurs scattered through the intrusive ClimaxGranitePorphyryand the intruded Idaho Springs Formation. Visitors can tour the surface workings during the summer months.

Molybdenum now ranks as the number one metal mined in Colorado. Over $105,000,000 of “moly” was mined here during 1969, almost all of it from the Climax Mine, the world’s largest single source of this metal. The Climax deposit is located high on the west slope of Ten Mile Range in central Colorado, about100 miles southwest of Denver. It is in the central part of the Colorado mineral belt, near the MosquitoFault, a prominent structural feature which extends about sixty miles along the north-south trend of the mountains. Rocks on both sides of this fault are intruded by Tertiarygranitedikes, sills, and stocks. The Climax Mine is in astockjust east of the fault, near the axis of a broadanticlinein Precambrian metamorphic rocks.

Ore minerals at Climax aremolybdenite,huebnerite, andcassiterite;pyriteis recovered also for the manufacture of sulfuric acid. The ore is very low in metal content, containing only one-third of a percent of molybdenum, 0.005% tungsten trioxide, and 0.0001% tin. The great size of the ore body and efficient recovery by modern methods make Climax a profitable mine, however. Production has risen each year since the mine began operation.

Urad Mine near Berthoud Pass is a newly developed near-surface molybdenum mine similar to Climax. Nearby at the Henderson Mine the ore body is more than half a mile below the surface of the ground.

Over a large area of thePlateauProvince in western Colorado, Mesozoicsedimentary rocksare locally stained bright yellow, orange, or green. Such staining suggests mineralization, and radioactive compounds were recognized here before 1900. At that time, however, there was little or no market for them or for the vanadium frequently associated with them. When Marie Curie required radium for experiments with her newly discovered element, the raw materials were sent from western Colorado; by and large, though, production of radium from these ores was prohibitively expensive.

In 1905, vanadium was found to be effective in toughening steel. The Vanadium Corporation of America was formed to mine the Colorado ore. This company mines a rich zone in the Jurassic Entrada Sandstone, where vanadinite occurs with carnotite and other uranium ores. In the early days of vanadium mining, the uranium ores were discarded with otherganguematerials; now, of course, uranium is produced from them.

Since 1945, uranium production has been an important Colorado industry; in 1969 about $17,500,000 worth was produced. Uranium occurs in the state in two very different situations. In thePlateauProvince, where it was first discovered, it occurs insedimentary rocksas patches of pitchblende, carnotite, and a greenish yellow mineral called schroekingerite. It is most abundant in the Triassic Chinle Formation and the Jurassic Entrada and Morrison Formations, where it was probably deposited by downward movement of rainwater from overlying uranium-rich Tertiary volcanic rocks. Concentrations of uranium often occur in or near organic matter such as coal,fossilbone, or petrified wood, so mines tend to be located along rock layers carrying abundant organic material.

Another type of uranium ore is found in the Mountain Province.Veinsin Precambrian rocks of the Front Range and several other ranges contain pitchblende which seems to have been deposited by hot groundwater rising through broken and fissured Precambrian rocks. Often exceedingly rich, such ore is mined in the manner of most of Colorado’s metals. The Schwartzwalder Mine, a few miles northwest of Golden, has produced more ore of this type than any other mine in Colorado.

Petroleum and natural gas have been found in large quantities in the Prairie andPlateauProvinces in Colorado, as well as in smaller quantities in North Park in the Mountain Province. They generally occur in porous sandstone and limestone layers, where they have been trapped by overlying finer-grained, less permeable layers in or nearfoldsandfaults.

Several oil and gas seeps were found along the mountain front shortly after the arrival of the earliest settlers. Near Canon City, on Oil Creek, a plaque commemorates the first production:

Oil Creek—site of the first oil well in the west—second place in the United States to produce petroleum from wells. In 1862 ... A. M. Cassedy drilled an oil well 50 feet deep. By February, 1863, production was one barrel a day. Later, several thousand gallons of petroleum were produced by primitive methods, and kerosene and lubricating oil were shipped by ox team as far as Denver and Santa Fe.

About twenty miles to the southeast, near Florence, the Cretaceous Pierre shales were drilled in 1876. Oil was found in a system of intersecting fractures andjoints. Some of the early wells in the Florence field are still producing, making this Colorado’s oldest and longest producing field. It has yielded more than 10,000,000 barrels of oil.

Small quantities of oil have been produced near Boulder since about 1900, also from Pierre sandstones and shales. In this area, wells were located by “dowsing” or “witching,” as was fashionable at the time. Several old rigs can be seen near Boulder Reservoir. As at Florence, oil has been trapped in fractures of otherwise dense and impervious shale. Some gas is produced and is used by local farms.

More recently, oil was found far beneath the surface in the northern part of the Prairie Province. Here, in the Denver Basin, oil is produced from several levels in the Dakota Sandstone. The oil has accumulated in lenses of beach sand deposited along the shoreline of the Cretaceous sea. The general trend of the shoreline, and of the oil fields, is northeast-southwest. The shore appears to have been similar to Georgia’s present coastline: a swampy tidal zone separated from open sea by lagoons, sandy bars, and clean sand beaches.

Individual oil pools in the Denver Basin are small, but there are many of them; they lie nearly a mile below the surface, under much of Morgan and Logan Counties and adjacent parts of Nebraska. Exploratory and development drilling keeps total oil production at about 50,000 barrels a day. Oil and gas produced here is piped to Denver and other Colorado cities.

In southeastern Colorado, oil and gas occur in late Paleozoic limestones and sandstones similar to those which outcrop at the edge of the Wet Mountains. Prospecting by geophysical methods and by drilling has revealed several small, rich accumulations, one of which is thought to contain about 30,000,000 barrels of oil.

The Rangely field, in northwestern Colorado, is the most productive field in the state. Located in the northeastern part of the Uinta Basin, it is an outstanding example of an anticlinal field, where oil is trapped in a large, gentledome. The shape of the dome shows up well on the surface; rock layers can be seen dipping outward in all directions from the town of Rangely. Oil was found by drilling on the crest of the dome. At first, oil was produced from fractures in the Cretaceous Mancos Shale at less than 1,000 feet depth. Later, deeper drilling showed that oil had also accumulated in the Permian Weber Sandstone, at 5,000 to 7,000 feet. At present this field is producing about 28,000 barrels of oil a day, but the figure is dropping each year as the field is depleted.

Oil and gas are produced in southwestern Colorado from the eastern edge of the Paradox Basin and the northern edge of the San Juan Basin. In the Paradox Basin, oil comes from Pennsylvanian limestone mounds orreefs. Production in the Colorado part of the basin has been at most a few thousand barrels per day; more is produced in adjacent Utah. In the San Juan Basin, gas and oil are trapped in thin porous layers of Cretaceous and Pennsylvanian sandstone, between impervious layers of shale. Most of the production is in New Mexico, although some oil comes from the Colorado part of the basin.

The greatest known potential oil resource in the world lies in the oil shales of western Colorado. The richest of these shales cover an area of 1,600 square miles north of the Colorado River, south of the White River, and just east of the Colorado-Utah line. The oil shales are part of the Tertiary Green River Formation, which extends over much of northwest Colorado, northeast Utah, and southern Wyoming. Oily material calledkerogenis locked in these rocks, too solid to flow out of the fine pore spaces of the shale. To free it the shale must be mined, finely crushed, and heated until the kerogen converts to liquid oil. This is an expensiveprocess, and as yet production of petroleum from the oil shale has not been possible at a cost which will compete with production of oil and gas from wells. The United States Bureau of Mines, as well as a number of oil companies, have sought for more than fifty years to discover a less expensive method for extracting oil from the shale. No doubt at some time in the future a competitive technique will be developed, or a growing shortage of other oil will bring world prices to a level with which present production techniques can compete.

Oil and gas production in Colorado is decreasing at present, even though great efforts are being made to find new oil pools. Petroleum prospecting and wildcat drilling are carried out in as yet unproductive basins in thePlateauProvince, in intermontane basins such as the San Luis Valley, and on the Plains. Known reserves will continue to provide the state with significant income for many years to come, and if oil shale recovery becomes profitable. Colorado’s hydrocarbons will become the most prominent of her commodities.

Coal resources of Colorado amount to about 60 billion tons. Only one per cent of this has been mined. Thousands of tons are now being produced daily from large mines in central, southern, and northwestern parts of the state.

Colorado’s coal deposits were formed during late Cretaceous and early Tertiary time, when seas were receding from this region and the land was rising. They represent accumulations of leaves and other plant material in swamps and flood plains similar to those now found in the delta of the Mississippi River and in the swamps of southeastern United States. Almost all Colorado coal is bituminous or soft coal.

Coal was recognized early in Colorado history by settlers along the mountain front, and was mined west and north of Denver in the 1860s. Several large underground mines still operate in this district, supplying local power plants, but production does not compare with that of the Walsenburg-Trinidad area in southern Colorado or the Hayden area in northwest Colorado.

The Walsenburg-Trinidad region, part of the Raton coal field, has produced coal since the building of the Santa Fe Railroad in the early 1870s. For many years coal from these mines moved the Santa Fe trains and many of the numerous smaller railroadsthat served Colorado’s cities and mining camps. The location of the mines helped to determine the location of the Colorado Fuel and Iron Company smelter in Pueblo. Now, most southern Colorado coal is used to produce electric power. Many small mines, miles away from the power plant west of Trinidad, are deserted.

A large coal-burning power plant has recently been built between Hayden and Steamboat Springs, just west of the Yampa River. Here, some of the extensive coal deposits can be seen in road cuts along U. S. highway 40. Until conversion to diesel fuel became almost universal in North American railroads, mines of this district produced coal for locomotives.

In the heyday of the gold and silver mines, coal was also mined near Coalmont, in North Park, and Como, in South Park. Coal from these areas was used for fuel in nearby mining towns and ranches, and for the narrow-gauge railroads that penetrated the mountains here.

At Anthracite, near Crested Butte, high-grade anthracite coal was mined for a time. Identical in origin with other Colorado coal, the anthracite of this region was hardened by heat and pressures from Tertiary igneous intrusions forcing their way into localsedimentary rocksduring post-Cretaceous mountain building.

A multitude of other coal camps are scattered about Colorado: Cokedale, Delcarbon, Coaldale, Roncarbo, Carbondale, and Cardiff stand out because of their suggestive names. These early small camps are, like their metal-mine cousins, largely deserted today.

Sand, gravel, and crushed rock rate high among geologic products in Colorado; more than $27,000,000 worth of these materials were produced in the state in 1969. Highway and construction activities have brought recent expansion in the number and size of quarries and gravel pits. Increasingly, Coloradoans are insisting that quarries and pits be excavated only where they will not mar the natural beauty of the landscape, and many old pits are now being filled in. Unfortunately, the scars left by some quarries—such as that on the Rampart Range near Colorado Springs—are difficult to erase.

Clay of good quality occurs in Cretaceous deposits in many parts of Colorado, most frequently in the Dakota or LaramieFormations. In the area around Golden, the Coors Porcelain Company for many years mined clay for use in pottery and low temperature ceramic ware. Scars from this mining can be seen along the mountain front north and south of Golden, and deep clefts within the town, just west of Colorado School of Mines, testify to the amounts of clay that have been removed. Colorado clay is not pure enough to be used in high temperature ceramics, and the present use for it is in the manufacture of common tiles and bricks.

A recent development in Colorado is the use of Cretaceous Pierre shales in manufacturing lightweight aggregate for building. The shale is mined between Golden and Boulder, near Colorado highway 93. In the nearby plant, it is pulverized and then heated in a large rotating cylinder until the surface of each particle fuses. Then the particles are quickly cooled. The resulting product is much like cinder, light in weight and yet strong. It can be mixed with cement for use in construction work requiring a great strength-to-weight ratio, or made into concrete blocks.

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