Mine. County. Rock. Sulphur Creek Colusa Serpentine. Abbott Lake Shale-serpentine. Great Western Lake Serpentine. (?) Ætna Napa Sandstone. Corona Napa Sandstone-serpentine. Aat Hill Napa Sandstone. New Almaden Santa Clara Shale-serpentine. Barton Siskiyou Shale-sandstone. Cinnabar King Sonoma Sandstone-serpentine. Altoona Trinity Porphyry-serpentine. A study of the foregoing shows that serpentine is almost as intimately connected with quicksilver as is quartz with gold, or granite with tin. These are the things that prospectors should make a note of. With the great increase of gold mining and the limited store of cinnabar that is available that ore seems certain to rise in value before long. Mica. The value of Indian mica varies from 90¢ a pound for sheets 4 in. × 1 in. to $13 a pound for sheets 10 in. × 8 in. The white mica in large sheets is valuable. The amber-colored, and spotted, are used for insulating purposes in electric plants, while the coarser sorts are ground and used as lubricants, or in fire-proof paint manufacture. Nickel. This ore is never found in metallic form, but always in combination. Pyrrhotite, or magnetic pyrites, is the source of about all the nickel of commerce. This ore has been already noticed under iron. Rare but valuable ores of nickel are millerite, nickelite, glance, and nickel bloom. Per cent. nickel. Millerite NiS 64.4 Niccolite NiAs 44.0 Some of the nickel of commerce is derived from nickelliferous pyrrhotite. Petroleum. Crude petroleum is never found in metamorphic or igneous rocks. The stratified rocks of the Devonian, Carboniferous and Cretaceous ages are most likely to hold it. The crude oil is almost black, and consists of about 85 per cent. of carbon, and 15 per cent. of hydrogen. A long iron-shod stick is all the prospector requires to take with him in his search for surface indications of oil. The warmer the day the easier the search, as the oil rises to the surface of the streams, and is found in greater quantities than on cold days. Oil existing in the lower rocks ascends through them until it accumulates under some layer that will not let it pass through. In this condition deep boring finds it, the rod usually tapping gas first. Petroleum may be noticed oozing out of gravel banks, or floating as a scum on the surface, whenever abundant. It has been found in rocks of widely different age, from extremely ancient formations to some that did not precede man by so very long, geologically speaking.