154 UPPER, OR FLINTY CHALK. 



On this subject Professor Buckland remarks, that " although, in the 

 present compact state of the matter of flint, it is not easy, though pos- 

 sible, to force a fluid slowly through its pores, yet it is probable that be- 

 fore its consolidation was complete, it was permeable to a fluid whose 

 particles were finer than its own ; and that the particles of chalcedony, 

 whilst yet in a fluid state, being finer than those of common flint, did thus 

 pass through the outer crust to the inner station they now occupy; 

 where they also allowed a passage through their own interstices to the 

 still purer sihceous matter, which is often crystallized in the form of 

 quartz in the centre of the chalcedony, and so entirely surrounded by it, 

 that it could have no access to its present place, except through the sub- 

 stance of the chalcedony, and the flint enclosing it *." 



3. Calcareous spar. This mineral is abundant in the fissures 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 modifications generally 

 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, cohimnar, 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 mani- 

 festly 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-j-. 



Of the columnar crystals, some fine specimens were brought to view 

 by the tremendous fall of the cliffs near Beachy Head, that happened a 

 few years since. These occurred in large masses of a yellowish colour, 

 and the crystals when detached were semi transparent ; Plumpton Plain, 



* Geological Transactions, Vol. iv. p. 419. 



\ From the correspondence of J. Hawkins, Esq. 



