§4. 



CHALK DOWNS. 



299 



The upper division of the cretaceous system is composed 

 of beds of a very peculiar white limestone, varying in thick- 

 ness from a few inches to several feet. In the Upper Chalk 

 there are numerous parallel rows of siliceous nodules, 

 termed flints, disposed in irregular distances from each 

 other ; and in some places sheets or seams of flint are 

 intercalated between the chalk strata, and extend over 

 considerable areas. The chalk is also traversed by vertical 

 and diagonal veins of flint. The only metallic substances 

 are oxide of manganese, in the state of dendritical or 

 arborescent markings ; and oxides and sulphurets of iron. 

 Iron -pyrites frequently constitutes the casts of shells, echini, 

 &c. — the surface of the mineral retaining sharp imprints 

 of the external characters of the original. 



The white chalk is composed of lime and carbonic acid ; 

 and a microscopical analysis proves it to be a mere aggre- 

 gation of shells and corals, so minute that upwards of a 

 million are contained in a cubic inch of the rock ; the 



Lign. 50.— Chalk-dust; highly magnified, seen by reflected light. 



a, a. Rotaliae. b. Textilaria?. 



{From " Thoughts on a Pebble.") 



amorphous particles appear to be the detritus of similar 

 structures, and not a chemical precipitate. The appearance of 

 a few atoms of chalk -dust, under a highly magnifying power, 

 is shown in this figure {Lign, 50). These organisms are the 



