Page:VCH Northamptonshire 1.djvu/72

 A HISTORY OF NORTHAMPTONSHIRE in long thin beds in the Estuarines and Forest Marble ; masses of radiated spar in the Northampton Sand (Northampton) ; aragonite in the North- ampton Sand more rarely ; and stalactitic limestone in joints and crevices of limestone rock. Calcium Sulphate occurs as gypsum or selenite, hydrous calcium sulphate, in the characteristic transparent crystals, and in larger elongated masses traversed by a central vein of dirt (twin crystals). The Upper Lias in particular contains this mineral. Allophane, or a mineral nearly allied to it in composition, has been found in crevices of the Northampton Sand at Northampton.^ ' Samuel Sharp, ' The Oolites of Northamptonshire,' pt. i.. Quart. "Journ. Geol. Soc. (Aug. 1870), p. 354. 40