316 



THE WONDERS OF GEOLOGY. Lect. IV. 



tion, and became extinct, or at least no longer inhabited 

 the same parts of the ocean at a subsequent epoch ; while 

 other forms appear for the first time. 



The state of preservation in which the fossils occur, 

 varies according to the lithological composition of the strata. 

 The shells, corals, echinites, &c. of the white chalk, are 

 generally transmuted into carbonate of lime having a 

 spathose structure ; their cavities are commonly filled with 

 chalk, flint, or sulphuret of iron ; in many instances they 

 are hollow, and lined with crystals of carbonate of lime. 

 The softer zoophytes are often silicified, and there is 

 scarcely a flint nodule in which their remains may not be 

 traced. The bones of the reptiles and fishes, and the cover- 

 ings of the crustaceans, are in a friable state, and stained with 

 sulphuret of iron. The teeth and scales of the fishes possess 

 a high polish, and are more or less permeated by ferruginous 

 infiltrations. Wood occurs in the state of lignite in the 

 clays and sands, and in brown friable masses in the chalk, 

 which quickly decompose upon exposure to the air ; but 

 when enveloped in flint, the structure is silicified, and well 

 preserved;* like the fossil wood of the tertiary, it has 

 evidently been drifted, and is perforated by teredines and 

 other boring animals ; the fissures are often filled with 

 brilliant pyrites. 



In the Galt> the nacreous covering of the shells is com- 

 monly preserved, and the ammonites and nautili of Folk- 

 stone rival in beauty the shells of the London clay, and 

 like them are subject to decomposition. 



The Gveensand fossils in the cherty sandstones are often 

 silicified ; and the whetstone pits of Devonshire are cele- 

 brated for the beauty and variety of the chalcedonic shells, 

 star-fish, &c. with which they abound. In the sands, lime- 

 stones, and argillaceous strata, the shells are for the most 

 part well preserved. 



* Medals of Creation, vol. i. p. 168. 



