Vol. 67.] 



THE AVON TAX OF BTJRRINGTON COilBE. 



361 



(B) Siliceous Rocks. 



Chert occurs on three horizons : — 



(a) At about the middle of Zi. 



(b) In the lower part of C 1( where the development is very strong. 



(V) At about the middle of S.„ where again the development is very 

 strong. 



The chert, in most cases, is clearly not an original deposit, but 

 produced by the subsequent alteration of the limestone. The 

 corals in S,, C L , and Z y are often replaced by chert, and this 

 sometimes takes place with the crinoids in Z, "while the matrix is 



FL? . 10. — Looking up the valley of the western twin-stream. 







& Zauc4cto ZVrx, 



, 







'-*•*'" 



rjf. 





S. H. E. photogr. 



comparatively little affected. On the other hand, it is frequently 

 observed, though less markedly at Burrington than at some other 

 localities in the Mendips, such as Waterlip and Windsor Hill, that 

 in some crinoidal limestones the matrix has been entirely replaced 

 by chert; while the crinoid-stems have not been affected, and, having 

 been subsequently removed by solution, are now represented by 

 cavities. As a rule, calcitic organisms, such as crinoids, show greater 

 relative resistance than arragonitic organisms, such as corals, 

 whether the metasomatic change is one of dolomitization or of 

 silicification. 



With the exception of these cherls, siliceous rocks are not met 

 Q. J. G. S. No. 267. 2 c 



