88 
UPPER CHALK. 
rounded by it, that it could have no access to its 
present place, except through the substance of the 
chalcedony, and the flint enclosing it.”* 
8. Calcareous spar. This mineral is abundant 
in the Assures and hollows of the chalk, and forms 
the constituent substance of the shells and echinites. 
It is of various shades of amber colour, brown, and 
pearl white ; the variety into which the shells and 
echinites are converted is opaque, and has an 
oblique fracture. The other modiflcations gene- 
rally possess some degree of transparency ; in some 
of the larger bivalves, of the genus Inoceramus, 
the structure is fibrous. 
The crystals of carbonate of lime are of various 
forms : the most usual are the rhomboidal, colum- 
nar, and acicular. The first occurs abundantly in 
cavities in the chalk, immediately beneath the turf, 
on Plumpton Plain ; and it is worthy of notice, 
that the hollows it occupies have manifestly been 
formed subsequently to the consolidation of the 
chalk. In Western Sussex, branched cavities in 
the chalk, apparently occasioned by the decay of 
ramose zoophytes, are incrusted by this variety of 
calcareous spar.t 
Of the columnar crystals, some fine specimens 
were brought to view by the tremendous fall of 
the cliffs near Peachy Head, that happened a few 
years since. These occurred in large masses of a 
yellowish colour, and the crystals when detached 
were semitrans])arent ; Plumpton Plain, Alfriston 
* Geological Transactions, vol. iv. p. 41 a 
i' From the corresimiulencc of .1. Hawkins, Es(j. 
