From what we have said, it now seems probable that the great majority of dolomitic limestones owe their magnesium to substitution from without. Direct precipitation of dolomite has, however, been invoked to account for several cases of Permian age, such as the Magnesian Limestone of the county of Durham. Near Sunderland, this rock is greatly modified, containing ball-like and other concretions, associated with frequent cavities. Traces of the original bedding remain, running through the concretions, and marine fossils are abundant. Conybeare and Phillips, so far back as 1822, stated that the nodules were devoid of magnesia, though formed in a magnesian rock. In spite of this, these objects long appeared as dolomite in collections. E. J. Garwood[17] showed conclusively that they resulted from the concentration of calcium carbonate in a concretionary form. The process whereby a dolomite may thus revert towards the ordinary limestone condition, with removal of magnesium in most cases, has been styled "dedolomitisation." Water containing calcium sulphate after passing through a dolomite is found to carry magnesium sulphate by a chemical exchange. Skeats[18], moreover, points out that, under a pressure of five atmospheres the magnesium carbonate of dolomite becomes more soluble than the calcium carbonate in fresh water containing carbon dioxide. The ordinary relations are thus reversed under pressure, and a cause of dedolomitisation may be indicated. Under the influence of contact-action from igneous rocks, dolomite may separate into calcium carbonate, magnesium oxide, and carbon dioxide. The magnesium oxide takes up water and yields the flaky colourless mineral brucite. Where silica is present, either as an impurity in the dolomite, or introduced from an invading siliceous magma, magnesium and calcium silicates may be built up[19]. Olivine thus arises, and, on becoming hydrated and passing into serpentine, stains the rock in various shades of green. The calcium carbonate crystallises as a ground of granular calcite, and the whole mass becomes a handsome _Ophicalcite_, or serpentinous marble. The famous rock of Connemara, used in polished slabs, has arisen through contact with intrusive diorite. Dolomitic limestones are liable to decay rapidly in towns, owing to the formation of magnesium sulphate, which, as shown above, is even more soluble in water than is the accompanying calcium sulphate. In the country, the crystals of dolomite resist ordinary weathering by the carbon dioxide of the rain-water better than those of calcite; and the rock thus becomes loosened through the loss of one constituent, and crumbles into a dolomite sand[20]. Compact dolomites, however, have furnished some excellent building-stones for country use, since here the more resisting mineral forms the bulk of the rock.