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 



