The basic igneous rocks, from which the platinum is derived, are exposed in the form of discontinuous elliptical outcrops near the summits of the Urals, particularly on the western side of the mountains. These outcrops attain larger dimensions in the northern and central Urals than further south. The process of concentration of platinum in the gravels has clearly extended over a very long period of time, and it is probable that the richer gravels have been reconcentrated, perhaps several times[52] p. 299. Platinum nuggets are rarely found, but three of large size were discovered in the Nizhni-Tagilsk district, their weights respectively being 25½ lb., 21 lb., and 11½ lb. Duparc, in a brief description of the geology of the deposits, states that they are essentially of magmatic origin. The structure of the rock is in the nature of concentric bandings; the felspathic rocks at the outer edge gradually grade into the intermediate stage of pyroxenes, until the central dunite is reached, composed of olivine and chromite. The richness of the gravels is in proportion to the size of the dunite deposits, and to the extent of erosion of these rocks. According to Duparc, platinum ore derived from a pyroxenite source usually contains high percentages of platinum and palladium, but low percentages of osmium and iron. The same writer estimated in 1916 that the reserves were sufficient for about twelve years, provided that the same methods of working and rate of extraction were employed during that period[53]. The amount of the platinum in the wash is very variable. The average yield was formerly over ½ oz. per c. yd., but latterly, owing to the gradual exhaustion of the richer deposits, the average returns have not exceeded from 2 to 3 dwt. per c. yd. The crude metal assays about 83 per cent. platinum, from 5 to 7 per cent. osmium and iridium, with small amounts of ruthenium, palladium, and gold, and about 10 per cent. impurities, mostly iron and copper[54]. The thickness of the pay gravels varies from 3 to 6 ft., and about 4 ft. may be taken as the average width, the overburden running from a few to over 60 ft. The extent of the productive area is about 170 sq. miles, and from 15,000 to 20,000 miners were employed before the war[4] p. 558. It has been noticed that the platinum deposits are characterized by their uniformity, of course not being so sinuous in direction as gold deposits.