depth in the rocky barriers. The Gunnison rises in the Sawatch and Uncompahgre mountains, and pursues a N. W. course, through a continuous series of mountain chasms, to its confluence with the Grand; it has numerous tributaries. The Rio San Miguel and the Dolores rise in the San Miguel and La Plata mountains, flow N. W., and after uniting fall into the Grand. The Rio Grande rises in the S. W. part of the territory, E. of the Sierra La Plata, flows E. about 150 m., then bends abruptly, and pursues a S. course through the middle of the San Luis valley. Along the S. W. border of the territory are numerous streams which flow S. to the San Juan in New Mexico. On the plains many of the smaller tributaries of the Arkansas and the Platte disappear in the sands during the greater portion of the summer.—E. of the main range of mountains, a portion of the country N. of the divide has been to some extent geologically examined. Denver is situated on the tertiary rocks which contain the coal beds of the west. The rocks here are thickly covered with superficial drift. Passing S. up the valley of the South Platte, the tertiary sandstones are occasionally exposed in the banks of the river. About 12 m. S. W. of Denver are some remarkable soda lakes, resting on middle cretaceous rocks. From these lakes to the great divide the cretaceous and tertiary beds are concealed by superficial gravel and sand. On each side of the divide, beds of whitish-yellow and reddish sandstones appear, holding a nearly horizontal position. In the N. part of Colorado, near the E. base of the mountains, beds of tertiary coal have been found. The main range of mountains, particularly the gold and silver lodes, is composed of gneissic and granitic rocks. In the mountain valleys are immense deposits of modern drift. Bowlder drift is conspicuous in the mountains that wall in South park on the N. and N. W., while along the W. and N. sides appear lofty eruptive peaks, which seem to be old volcanoes. The mountains E. of the park have a gneissic and granitic nucleus. Within the park sedimentary rocks are found, and there are also salt springs and deposits of gypsum. The portion of Colorado W. of the main range of mountains forms part of the great volcanic basin which stretches S. into New Mexico, and N. W. into Utah and Idaho territories. In this region are many extinct volcanoes. The lava rocks which abound are not usually metalliferous, though they contain much mineral glass (obsidian). During the process of liquefaction which these rocks have undergone, vast areas, which now resemble lakes of black solidified sea water, have in some instances been submerged by the liquid overflowing from fissures hundreds of miles in length. The technical name of this formation is pedrigal, while the rocks are called malpais. The Rio Grande, from its source to beyond the limits of Colorado, flows through a pedrigal of extraordinary dimensions. In Middle park all the sedimentary rocks known in the country are found. Carboniferous beds are probably wanting, but the triassic, Jurassic, cretaceous, and tertiary are well developed. There are two groups of tertiary deposits: the lignite, or older tertiary, and the modern pliocene marls and sands. Grand river, just above the hot springs, passes through a high ridge of basalt, which has the lignite tertiary beds above and the cretaceous shales beneath. The tertiary rocks are of great thickness, and are composed mostly of fine sandstone and pudding stone. At the Grand cañon, just below the hot springs, the river cuts through a ridge of massive feldspathic granite for a distance of 3 m. between high walls.—Vast deposits of useful minerals of almost every kind occur in nearly every portion of Colorado. The most important of these are gold and silver, which are found in large quantities in a belt about 50 m. wide stretching N. and S. across the central portion of the territory. Gold occurs in lodes, or fissure veins, having a N. E. and S. W. direction, and in gulches or in placers; the latter being superficial deposits which have been washed from mountain summits and slopes to the plateaus, gulches, and valleys below. The veins occur in groups, often presenting the most complicated network on the surface. These groups are usually one or two miles in width and two or three in length, and there may be two or three distinct groups abreast of each other. The principal gold-bearing minerals are copper and iron pyrites. These mostly occur together; the latter, however, nearly always predominates, and is often found without the former. When both are present, the copper pyrites is always the richer in gold. These ores assay in bulk from $30 to $40 per ton. About 70 per cent. of the gold bullion is extracted from the ores of Gilpin, Clear Creek, Boulder, Park, and Lake counties, 50 per cent. or more being furnished by Gilpin county, the bullion shipped from which for the year ending July 1, 1870, amounted to $1,378,100. The deposits at the branch mint at Denver for the year ending June 30, 1872, amounted to $1,001,564 81, of which $16,336 54 were silver. The total deposits of gold which had been made at this office up to June 30, 1872, amounted to $5,552,371 69, of which $4,985,754 67 were the product of Colorado. According to official mint returns, the deposits of gold from Colorado at the United States mint, branches, and assay offices, up to June 30, 1872, have been as follows:
| YEARS. | Value. |
| 1859 | $4,171 70 |
| 1860 | 599,846 30 |
| 1861 | 2,091,197 17 |
| 1862 | 2,035,416 50 |
| 1863 | 2,893,336 87 |
| 1864 | 2,136,684 69 |
| 1865 | 1,622,249 45 |
| 1866 | 1,018,052 52 |
| 1867 | $1,026,276 83 |
| 1868 | 1,081,040 16 |
| 1869 | 1,652,492 21 |
| 1870 | 1,551,102 81 |
| 1871 | 1,495,035 66 |
| 1872 | 1,176,518 09 |
| Total | $20,338,420 96 |
