RESIDUAL ORES The residual deposits always attain their greatest develop ment in those countries that have been unsubmerged for a vast period of time; and where the weathered products have neither been pared off by glacial erosion, nor the solid rocks protected by superficial accumulations. Under especially favourable conditions of climate and topographic relief, disintegration, accompanied by the removal of soluble material, has progressed to many hun dreds of feet below the surface, the whole of this zone consisting then of soft decomposition products that bear no likeness to their parent rock, although certain original structures, such as bedding and foliation, may be retained.
The materials composing the bulk of the residual deposits are gravel, quartz-sand and clay or lithomarge (an amorphous hy drated aluminium silicate, having the composition of kaolinite, viz.: and more exceptionally the mixture of hydrated oxides known as laterite. This latter material occurs in tropical and semi-tropical countries like India, the Malay Penin sula, the Dutch East Indies, South America, and East, West and South Africa; in fact, it appears to be one of the commonest residual products of ferruginous and aluminous rocks under suit able climatic conditions, A great number of analyses of laterite have clearly established the fact that it consists to a large extent of a mixture of oxides and hydrated oxides of iron, aluminium, titanium and manganese: they show also that the water is chiefly combined with the alumin ium oxide, the iron oxide being mostly present in the anhydrous or slightly hydrated condition (haematite or turgite). The ratio between the iron oxides and the aluminium oxides may vary to any extent, so that, while some laterites approximate in composi tion to high-grade iron-ores, others are almost pure bauxite which is the chief source of the metal aluminium. When manganese is present, it usually occurs in nodular concretions of psilomelane, wad or pyrolusite, which are sometimes sufficiently abundant to constitute ores of manganese. Mixed with these oxides in varying
proportion are a lithomargic clay and quartz sand.
There has been much discussion and considerable divergence of opinion as to the origin of laterite; but all are agreed that it may result from the decomposition of a great variety of rocks, whether of igneous, sedimentary or metamorphic origin. The generally accepted view is that laterite is a residual accumulation produced by the removal in solution of combined silica, lime, mag nesia and the alkalies, assisted by metasomatic replacement with material brought from the underlying rock in periods of desicca tion by capillarity, aided perhaps by the rootlets of forest trees.
Laterite, when exceptionally rich in iron, is of considerable eco nomic importance. Such, for instance, are the iron ores of Cuba, which occur as residual mantles on the high-lying plateaux of Malaya and Moa. Similar deposits occur in the Philippines, in Borneo, and in West Africa. The iron ores of Bilbao in North west Spain are also in part residual deposits.
Workable manganese deposits are nearly always residual con centrations from manganese minerals sparsely distributed through igneous rocks and crystalline schists. Deposits of this character are widespread in India, United States, Brazil, West Coast of Africa and many other places.
The nickel deposits of New Caledonia are of residual origin, the nickel ores being hydrated silicates (garnierite and genthite) occurring as earthy and brecciated masses in a ferruginous and clayey mantle overlying serpentine and peridotite.
Residual gravels containing tinstone and wolfram occur on Bodmin Moor in Cornwall. Others containing galena have been profitably worked in North Wales and again others, containing zinc ores, have been exploited as a source of that metal in Galicia. They have originated by the weathering of veins in limestones, chemical action on the soluble limestone having outpaced the mechanical erosion of the resistant minerals of the veins.