CHALK. 53 



seams between the strata. This substance does not seem to have 

 belonged to the chalk beds originally ; but to have been deposited by- 

 water percolating through the numerous fissures of the loose chalk, 

 and carrying with it a solution of chalk, mixed with mud washed 

 down from the alluvium. Some have called this m?ir\ fullers earth. 



Iron-ore is a mineral that occurs both in the white and the grey 

 chalk : but with this difference, that in the latter it occurs only in the 

 form oi pyrites, in small nodules of crystals, having a beautiful green- 

 ish yellow metallic hue; whereas in the former, the ore is in nodules 

 or small masses of a botryoidal form, having a ferruginous colour 

 without, and a dull iron colour within. This ore may be called a 

 kind of coarse Jirematite, as each protuberance of the botryoidal nod- 

 ules, when broken across, presents the appearance of rays diverging 

 from the centre. The external surface of some of the nodules seems 

 like a collection of imperfect crystals, with numerous angles, points, 

 and facets,. having a lustre approaching to metallic. In others there 

 is a rusty coating, the surface being deeply oxidated; and the rust 

 not only covers the surface, but penetrates into the pores or cracks 

 occasionally found in the nodules. Even the pyritous crystals in the 

 grey chalk, often acquire a slight coating of rust, when long exposed 

 to the action of the air and water. These crystals do not appear to 

 exist in the red chalk. 



In the upper chalk we also find, in small cavities, an ochreous 

 or ferruginous substance, apparently the same with that which consti- 

 tutes the coarse haematite nodules. Whether this substance has re- 

 sulted from the decomposition of such crystallized nodules, or has 

 never been crystallized at all, we cannot determine; but as nodules 

 are often found in a half decomposed state, the former appears most 

 probable. Wherever this ochreous substance is found, and indeed 

 wheiever these nodules are imbedded, the chalk is tinged with brown 

 or yellow, around the cavity or bed. Hence the white chalk abounds 



o 



