80 THE WONDERS OF GEOLOGY. Lect. I. 



and this large conical mass, which has been cut through 

 and polished to show its structure, was dug up on the 

 summit of Alfriston Hill, in Sussex, and must have been 

 formed in some fissure or cavern in the chalk, of which no 

 traces now remain. 



38. Consolidation of loose sand, &c. — The changes 

 effected by this process in strata composed of loose mate- 

 rials, are of still greater importance ; for by an infiltration 

 of crystallized carbonate of lime, sand is converted into 

 sand-stone, — fragments of soft chalk are transmuted into a 

 solid rock, as in the Coombe-rock of Brighton, — and accu- 

 mulations of beach, and gravel, into a hard conglomerate, 

 as in this example of the ancient shingle bed of the cliffs, 

 at Rottingdean ; — shells, into a building stone, as in this 

 mass from Florida, — and the detritus of shells and corals, 

 into limestone, as in these specimens from Bermuda. By 

 this agency, the bones of animals are permeated with 

 calcareous spar, and their medullary cavities lined with 

 crystals of carbonate of lime : and clay, which has cracked 

 by drying, has its fissures filled up, and becomes consoli- 

 dated into those curious masses, called septaria, which, 

 when polished, form the beautiful table-slabs for which 

 Weymouth is celebrated. 



39. Destruction of rocks by carbonic acid gas. — 

 Although, in the instances above cited, water, by its com- 

 bination with carbonic acid, occasions the solidification of 

 loose and porous beds of detritus, yet the effect of this gas 

 on certain rocks is that of disintegration ; for by its solvent 

 influence on the felspar, granite itself is reduced to a friable 

 state, the quartz and mica, which with felspar constitute 

 that rock, being set at liberty. The disintegration of 

 granite, is a striking feature throughout extensive districts 

 in Auvergne, especially in the neighbourhood of Clermont. 

 In the ancient shingle of the cliffs at Kemp Town, near 

 Brighton, blocks of granite occur which may be crumbled 



