UPPER, OR FLINTY CHALK. 155 



Alfriston chalk- pit, and some other locaUties, have produced similar ex- 

 amples. 



Obtuse rhomboidal crystals, of great beauty, have been found in a 

 chalk-pit near Alfriston ; their colour is of a delicate pearl white, and in 

 their general appearance they resemble the double refracting spar of 

 Iceland, except in their inferior degree of transparency. The cavities of 

 echinites are sometimes lined with rhomboidal crystals of carbonate of 

 lime, disposed in hues parallel with the sections formed by the areae of 

 the shell ; and the inner surfaces of the terebratulae, are frequently frosted 

 over with drusy crystals of the same substance. 



4. Sulphuret of iron, or iron pyrites, in subglobular and irregular 

 masses, is very common in the upper chalk. The external surface of the 

 specimens is invested by crystals of a pyramidal, octaedral, or cubo- 

 octaedral form; and their interior exhibits a radiated structure, possess- 

 ing a brilliant metallic lustre. When broken and exposed to the action 

 of air and moisture, they undergo decomposition with great rapidity ; 

 and even in cabinets, frequently form an efflorescent sulphate of iron, 

 and crumble into dust. This mineral occasionally incloses flints, shells, 

 echinites, &c.* and frequently fills up the cavities of the latter. A 

 specimen in my possession, exhibits on the upper side, a sharp cast of the 

 interior of an echino-spatagus ; and its base is covered with an elegant 

 group of quadrangular pyramids, evidently the terminations of octaedrons, 

 with their inferior angles concealed. 



The lower beds of the flinty chalk in South-street, contain detached 

 crystals of sulphuret of iron, remarkable for their neatness and ele- 

 gant figure. They are usually regular octaedrons, having their planes 

 studded with small quadrangular pyramids ; but some examples occur in 

 which the sohd angles are replaced by quadrangular planes, forming a 

 crystal with fourteen sides. A specimen of the former variety is repre- 

 sented, tab, xvi. fig. 10. 



* A terebratula imbedded in the centre of a nodule of sulphuret of iron, is represented in 

 British Mineralogy, tab. 171. It was found in chalk by Mr. Weekes, of Hurstperpoint. This 

 gentleman has also a perfect pyritous cast of Pecten Beaveri, from Clayton Hill, 



X 2 



