8 ORE DEPOSITS. [part I. of cast-iron from these sands is rendered difficult by their extremely fine state of division, but they have sometimes been advantageously employed for the direct production of blooms in the open fire. The black sands on the coast of California are not unfrequently auriferous, and are sometimes washed for the gold which they afford. Nearly the whole of the gold produced in the Russian empire is obtained from placer washings, vein-mining being exclusively confined to the Ural Mountains, and is even there carried on upon a very limited scale. The gold-bearing alluvium of the Ural is sometimes a heavy clay, while in other cases it is made up of water-worn fragments of auriferous quartz, chloritic and talcose schists, serpentine, greenstone, &c. Remains of various extinct animals occur deep down in these gravels, usually in the vicinity of the bed-rock. They include bones of Mephcts primigenius, Bos aurochs, and Rhinoceros tichorhinus, which are likewise found in the gravels of western Europe. Some of the auriferous gravels of the Ural repose upon a water-worn bed-rock of hard highly-inclined crystalline limestone, believed to be of Silurian age; in other cases they lie on a talc schist, or on a soft granitic rock containing pyrites and but little mica usually known as leresite. The two last-named rocks are traversed by veins of auriferous quartz. Platinum generally occurs with gold in auriferous gravels, and is seldom found without that metal except at Tagilsk and Goroblagodatsk, in the Ural, where there is little or no gold. Platinum is obtained from placer diggings only, and has not been found to any considerable extent in situ, although grains of this metal are said to have been observed in the quartz of the mines of Beresovsk. In the districts in which platinum occurs unaccompanied by gold, the rocks in the neighbourhood of the deposits consist of serpentine and peridotite, while fragments of these rocks predominate in the sands and gravels. Chloritic and talcose schists, together with chrome iron ore, are to some extent present. From the constant occurrence of this metal in association with gravels mainly consisting of peridotite and serpentine, it is thought that platinum originally existed in the form of grains disseminated through these rocks. In addition to gold and chrome iron ore, platinum is often associated with iridium and iridosmine. Streamworks.—The detrital tin ore of Cornwall may be grouped under the following heads :— a. Tin ore forming a constituent of river gravels and sea-