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Show 1294 REPORT. OF THE CHIEF OF ENGINEERS. denudation on account of the many fissures in which the water and melted snow freeze, thus expanding the cracks till the rock splits ami falls. The difference of hardness in the baxalt is well shown in the northeastern side of Mount Tallac. Isolated crags and piunacles stand out boldly from the mountain, while the rock that formerly connected them with it has been worn away by the influence of the elements. A vast amount of talus lies at the foot of the eastern slope of Tallac. Seeds of the white- thorn and man-zauita bush have been scattered over this debris and r. akeu root in the rocky soil, thns forming a dense thicket impassable for pack- animals. The southwestern declivity of Tallac Peak slants gradually to Lake Gilmore, and is covered with nutritfons grass, to* get her with occasional clusters of trees except for a distance of about 300 feet from the summit. Ledges of blue basalt outcrop in many localities on the southern and western sloprs. The height of Tallac Peak is 9,732 feet. Lake Gilmore occupies the bottom of a basin with lofty walls, and is 1,333 feet below the summit of Tallac according to the mercurial barometer. The temperature of the water iu this lake is 50° F. The eastern slope of Mount Tallac has been grojved and polished by glaciers. I f. jund tine examples of gla< ial 6cratclus abont half a mile from the summit. In some places the face of the cliff is as smooth as if cut by a chisel. The southern side of this monntain is exceedingly steep, and nearly all of it is covered with rocky Mbris, while a solitary coniferous tree here and there breaks the monotony of the scene. Granite surrounds Tallac Peak on all Hides except on the northeast, where Lake Tahoe forms the boundary- line* A description of this mountain would be incomplete without a brief reference to the maguiliceiit view seen from the top. The view is as varied as it is interesting. On the one side the entire range of the western summit is visible, while on the other there is the broad expanse of Tahoe with the eastern summit beyond. Twenty mountain- lakes are in sight. Their mirror- like surfaces, reflecting the* blue sky form a pleasant contrast with the somber hues of the densely timbered ridges. The prominent features of the country lying in front of Tallac Peak, as fir as Emerald Bay, have already been mentioned. Near the head of this bay is a knob of granite called the Emerald Isle, which is 150 feet high and 300 feet long, and has the shape of a pear. There U a waterfall abont 50 feet in height, a quarter of a mile from Mr. Holl Aday's house. The canon in which the inlet of Emerald Bay runs is very picturesque. It is narrow, windy, and the walls are very steep. Climlvng up the canon for half a milo from the bay I conld catch a glimpse through The clusters of trees of several small cascades. At Emerald Bay and from this point northward the rock is gray granite to Sugar Pino Point. Near Rubicon Point the rock contains red feldspar. There are four more peaks belonging to the Tallac Peak Ridge, the northermost of which is capped by a sharp granite tuiret. This feature of peaks culminating in rocky tors occurs elsewhere iu the western summit. The ridge becomes very narrow at the head of tho creek emptying into Meigs's Bay. From this point to Blackwood Cafioa there is no ro ^ k in mta within from a half to one mile of the lake- shore. The monntain behind McKiu-ney'a Station is basalt, and the slopes are covered with a deuse growth of whiro thorn and mauzauita. Some mineral indications have been found in the ridge about 1} miles from McKinuey's and 1,000 feet above Lake Tahoe according to the aneroid. The ledge was discovered in July, 1876, and work was begun in September. It is claimed by Messrs. Niles, Bellinger, Sims, and Casey. The vein runs northwest and southeast. In September, 1870, two men were working at an opening in the ledge 7 feet long and 4 feet wide. No analysis has yet been made of the ore, but it is said to contain nickel. On the northeast side of this mountain is Quail Lake, a body of water baying an area of about an acre, and 400 feet above Lake Tahoe. Going northward, the rock is chiefly basalt as far as Blackwood Cailon. Bine basalt occurs at the head of this cafion. The Twin Peak ridge runs from this point north to Trnnkee Caftan. Twin Peaks arc just north of Blackwood Cafion, and consist of basalt and diorite. From the summit of these peaks the observer beholds many V- shaped canons and serrated ridges. Sjnie of the mountains have the dome structure, some a e sharplike a knife- edge, others are conical, or pyramid- shaped or have the forms of a mesa. Scattered among these picturesque ridges of granitic and basaltic rocks are a few lonely Alpine lakes in cup- like babius. Twin Peaks are 2,604 feet above Lake Tahoe. West Twin Peak is formed of grayish basalt. The greater part of it is composed of myriads of horizontal prisms averaging about 0 inches iu diameter, which decrease in size as the base is approached. At the junction of the East and West Twin Peaks the rock is dark- blue porpbyritic basalt with white crystals of feldspar. East Twin Peak consists of gray porous diorite. Tho whole ridge from Blackwood Canon to Tahoe City is basalt, of gray and blue colors. Very little granite is found north of this cafion, but south of it as far as Lake Valley, and frolii 200 to 500 feet from the lake- shore, many bowlders of this rock paving a rouuded form occur. The cafion of the Truckeo River is formed of basalt. At the begiuning of the canon, in leaving Lake Tahoe, the basalt is porous and slightly porphyritic. There is dvbris on the north wall of the canon, where several crags of basalt outcrop. About a mile from Tahoe City the Truckee River breaks throngh a mass of pudding-stone basalt, that is slightly ferruginous. Many conifers and some quaking- aspeus grow |