70 
on the occurrence of these pebbles in the chalk, in the second edition 
of his ‘ Elements of Geology,’ p. 394. 
Iron, in the form of sulphuret or pyrites, and in different degrees of 
oxidation, is the only metal found in the Chalk formation: I have 
casts of shells, star-fish and sponges entirely composed of it. Nodules 
of pyrites occur in all shapes and sizes; many of them when broken 
exhibit brilliant radiated crystallization; but this appearance, when 
exposed to the air, soon vanishes, for the surface imbibes the oxygen 
from the atmosphere, and decomposes in the form of sulphate of iron. 
Pyramidal crystals often fill the cavities of shells and cover large 
masses of this substance. Acute and truncated octohedral crystals 
are found single, or disposed in irregular ‘groups. Cubic crystals 
(though rare) I have found in the pits near Lancing ; and some speci- 
mens at Houghton assume the shape of the cockscomb crystallization. 
In breaking the balls of pyrites, a Terebratula or small shell is some- 
times found in the centre, forming the nucleus of crystallization ; and 
crystallized sulphate and carbonate of lime are also observed in some 
masses. 
