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CARIBOU-GRAND ISLAND DISTRICT It/2 miles south\vest of Eldora, is a large stock of man- zoniteporphyry of group 3 and a small stock of coalse]}• porphyritic quartz monzmnite of group 0 (pl. 7 and fig. 12). To the east and north of Eldora there are se+•eral east+vard-trending dikes of hornblende monzonite poI•- phyry and hornblende and biotite andesite, which form part of an.estettsive dike system that can be traced east fot• many miles. The best known veins of the district are about half a mile sotltlt of Eldora. They contain tellurides and have been described by Lindgren ° and Rickard.T They have a general east-west trend, are commonly 1 to 3 feet wide, and consist of sheeted zones containing several Ital•ro+c seams of ore. TJIe outcrops of the veins are very inconspicuous and are more likely to be indicated by depressions than by prominent outcrops. The valuable metals in the ore are chiefly gold and u little silver; the valuable minerals are tellurides of gold, of which sylvanite and petzite are believed to bc: the Inost important. The tellurides occur as small specks in afine-grained "horn" quarti or in greenish 1oscoelite, rarely as well-crystallized mittet•als. Sparse shall grains of pyrite are present in the altel•ed country rock. 1lfolybdenite is moderately abundant but is in most places extremely fine-grained and intergrown with barite, Its presence in dumps is indicated by deep-blue stains, which were thought by Lindgren to be i]se- mannite. The chief gangue minerals are barite, quartz, roscoelite, and hm•n quar[•r.. The horn quartz is the most abundant, but barite is also abundant. Roscoelite is very prominent in the ores of the Dfogul tunnel and the Enterprise mine. It forms dark yello\v-green masses intergrown with quartz or irregularly dis- tributed iu the ore. and in many places contains visible speck of the tellurides. Some of [he roscoelite is inter- groo'n +\•ith pyrite and may be surrounded and invaded by fine granular quartz with some adularia. 'hhe wall rock close to the veins is largely altered to sericite and contains disseminated pyrite. Vugs are fairly com- mon in the vein and are frequently occupied by cn•stals of gangue minerals. Lindgren believed that these de- posits were not formed at great depth and that the origina' surface could not have been much different from the ganera] ]erel of the peneplain marked by the high ridge lines of the region. A fe\v mines near Lost Lake, 21/z miles \vest of El- dora, have had a small output. 1ltost of the ore has come from the Nor\cay, .lta \~, and Revenge veins near the intersections of the northeastward-trending Nor- +\'ay and I+la R' with the north\cestward-trending Re- venge. (.See pl. 3.) Small bodies of high-grade tel• luride ore have been mined on these three veins, and ° LIn~Fren, \caldemnr. Come FoIJ and hm FSlen dcpoAits of Boulder Counq', Colo.: Econ. Ccolo6y. col. ^_. pU. 454-480. IOOi. ' Rickn rd, '1•. A., 1'he Veins of Boulder County and Rnlgoorlie: .\m fmt. )tin. EnF Trnns., vol. 33, aa. 587-568, 3003. 197 it is repotted that most of the output has come from tlm Na•\+•ay and i\fa \'V veins. The Revenge shaft +vas full of water in 1938 and had not been \vorked since 1914, but development +vas being carried at in the Norway tunnel. REPRESE\TATI OE ~IINES The Enterprise vein ° crops out at an nltitude of about 9,400 feet on the north side of Spencer -Iountain about. half a mile south of Eldora. It is developed by a shaft about 400 feet deep with 5 levels and about 1,200 feet of drifts, The i\Iogul and S\cathmore tunnels cut +chat is supposed to be the Enterprise vein at greater depth. The vein fissure according to Rickard is about 5 feet wide and is traversed by numerous small seams of darh- gray horn quartz. This quartz contained enough tel- luride, chiefly petzite, throughout a 2- to 21/•z-foot \cidth of vein to yield an average of 2 ounces of gold to the ton locally. About 1,800 tons of ore from the Etttel•- prise shaft, treated in the Bailey mill at Eldora in 169fi and 1897, is said to hove averaged about $10.80 per ton, mainly in gold. The Mogul tunnel; the portal of which is near El- dora station, extends southwest and south for more than 1,400 feet and cuts a Itltmber of easterly to nol•tll- easterly telluride veins, most of which are only slightly mineralized. It cuts the Enterprise vein 000 feet belo++• •.I-le outcl•op. Ore taken from the 1\Iogul tunnel in 1005 is said to have averaged about nn ounce of gold to the ton. Ore from this tunnel oll the Enterprise vein is said to have averaged behceen 1 and 2 omtces to the ton. Eluch sloping has been done along one of the telluride veins for a distance of about 400 feet. The Swathmore tunnel, a shat distance above Eldora, trends southward and cuts the Enterprise and three other veins. Ore in the bins showed horn quartz, fine- grained p}'rite, and some roscoelite. Tellurides m'e said to be present in nat•row seams of horn quartz one-six- teenth to one-ei 11th of an inch \eide cutting the roscoelite. CARIBOU-GRAND ISLAND DISTRICT The Caribou-Grand Island district lies in the \cest- central part of Boulder County, at an altitude of about :0,000 feet, at the head of Coon Trail Creek. It is about 20 miles west of Boulder and about 4 miles north- north\vest of Nederland, frmn \chich.it is easily accessi- ble by road. The rich silver ores of the district were discovered in 1S60 by 1Villiant \lartin, George Little, and Samuel Conger. Conger \cas better known for his discovery of the tungsten district to the east many years later. In 1310 a wagon road \cas constructed to the district and shipments commenced to Blackha\ck, 20 miles a\vay. Dlining developed rapidly during the nest Bnslln, G. S., and HIII, ]. \L, ou clt., p. 788. Llcm.