16 INTRODUCTORY OBSERVATIONS. pally by the agency of a genus of foraminifera called Nummulites. This rock has been used as a marble in Italy, though seldom occurring with a crystalline structure. Hydraulic limestone. Limestone which has the property of setting under water after calcination. This is considered to be due to certain proportions of silica and alumina, and perhaps of carbonate of mag nesia. These limestones are generally earthy, compact, and of a blue colour. They are derived from the Lias of England, the lower Carboniferous series of Scotland, and of Ballycastle, Co. Antrim, &c. Dolomite, or Magnesian limestone. A limestone containing a large proportion of carbonate of magne sia ; crystalline or granular ; generally of a yellowish colour, and weathering into a brownish powder with a sandy appearance; sometimes fossiliferous, and con taining various proportions of foreign matter. The dolomite of the north-east of England is one of the chief building stones of the country. Gypsum, or Alabaster. White, yellow, red, or mottled, crystalline, or granular sulphate of lime; occurring in beds, or lenticular masses amongst the sedimentary rocks ; often in connection with rock- salt.