There is however another cause which must have contributed to round the angles both of calcareous and siliceous fragments; and that is, their solubility in water; calcareous earth is perpetually found suspended in the waters which pass over it; and the earth of flints was observed by Bergman to be contained in water in the proportion of one grain to a gallon. Kirwan's Mineralogy, p. 107. In boiling water, however, it is soluble in much greater proportion, as appears from the siliceous earth sublimed in the distillation of fluor acid in glass vessels; and from the basons of calcedony which surrounded the jets of hot water near mount Heccla in Iceland. Troil on Iceland. It is probable most siliceous sands or pebbles have at some ages of the world been long exposed to aqueous steams raised by subterranean fires. And if fragments of stone were long immersed in a fluid menstrum, their angular parts would be first dissolved, on account of their greater surface. Many beds of siliceous gravel are cemented together by a siliceous cement, and are called breccia; as the plumb-pudding stones of Hartfordshire, and the walls of a subterraneous temple excavated by Mr. Curzon, at Hagley near Rugely in Staffordfshire; these may have been exposed to great heat as they were immersed in water; which water under great pressure of superincumbent materials may have been rendered red- hot, as in Papin's digester; and have thus possessed powers of solution with which we are unacquainted. 8. BASALTES AND GRANITES. Another source of siliceous stones is from the granite, or basaltes, or porphyries, which are of different hardnesses according to the materials of their composition, or to the fire they have undergone; such are the stones of Arthur's-hill near Edinburgh, of the Giant's Causway in Ireland, and of Charnwood Forest in Leicestershire; the uppermost stratum of which last seems to have been cracked either by its elevation, or by its hastily cooling after ignition by the contact of dews or snows, and thus breaks into angular fragments, such as the streets of London are paved with; or have had their angles rounded by attrition or by partial solution; and have thus formed the common paving stones or bowlers; as well as the gravel, which is often rolled into strata amid the siliceous sand-beds, which are either formed or collected in the sea.