CHALK AND FLINT. 179 



veins of the same substance. In the Lower Chalk 

 of England, flints are of very rare occurrence, 

 and in general are altogether absent. The greatest 

 total thickness of the white chalk strata is esti- 

 mated at 1000 feet. The only metallic substances 

 observable are oxide of manganese in the state 

 of dendriditieal or arborescent markings in the 

 chalk and flints, and sulphurets and oxides of iron. 

 The most common form is the sulphuret of iron, 

 or pyrites, in spherical nodules beset with crystals, 

 which radiate from the centre, and when broken 

 a shell or other foreign substance is very com- 

 monly found to be the nucleus. This substance 

 often forms the casts of echinites, terebratulse, 

 and other shells, the surface of the mineral retain- 

 ing sharp imprints of the original. The bones 

 and scales of fishes and other animals imbedded 

 in chalk, have invariably a ferruginous stain. 



Chalk and flint. — The pure white chalk is 

 composed of lime and carbonic acid ; it dissolves 

 rapidly in hydrochloric acid, and leaves only a slight 

 residuum, consisting of silex and organic matter. 

 A microscopical analysis shows it to be a mere 

 aggregation of shells and corals, so minute that 

 upwards of a million are contained in a cubic inch 

 of chalk; the amorphous particles appear to be 

 the detritus of similar structures. These organisms, 



