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6Hence, in places where the mountains are not earthy, but of stone, the force of the fire escaping through the chinks burns that which is soft and tender, whilst that which is hard is left. Thus the earth of Campania, when burnt, becomes a powder; that of Tuscany a coal. Both of these are of great use in building, one species being very serviceable in land works, the other in works under water. In Tuscany, however, the quality of the material is softer than sandstone, but harder than earth; and from its entire subjection to the action of the sub-existing fire, it becomes that sort of sand which is called carbunculus.
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