236 M. B RONGJI I ART on the importance 



surface, often dips beneath it. This rock is the chalk. My 

 object is to shew the organic remains contained in it offer 

 characters sufficient for recognising it in situations Tery 

 distant from each other, when those drawn from its consis- 

 tence, stratification, colour, &c. have disappeared, and when 

 its superposition is either obscure, uncertain, or difficult to 

 be recognised. 



It should be recollected, that the mass of rock referred to 

 the cha!k formation, is difided into three sub-formations ; 

 the upper or white chalk ; the middle, which is grey chalk 

 or craie-tufau ; and the inferior, or chalk mixed with green 

 grains, which M. Bertliier has determined to be silicate 

 of iron (fer silicate) with water, and which I shall name, 

 considering it ao a mixed rock, glauconie crayeuse. It is 

 the green sand of the English geologists.)* 



These three di?isions of chalk contain fossil organic re- 

 mains, which are generally different in each sub-formation, 

 tut there are at tho same time some which are common toj^all. 



§ I. y»lue of Zoological characters in geology. 



Among the different chalk rocks I am about either io cite 

 or describe, many will without difficulty be regarded as 

 belonging to this formation ; some are even generally recog- 

 nised to form part of it j io these last I shall only add zoo- 

 logical proofs to the geological resemblances that have already 

 been established. 



I shall also refer this formation to places in which until 

 lately, chalk has not been recognised, in which this rock 

 is so much disguised, that I shall have some difficulty in 

 causing its analogous formation to be admitted with the in- 

 ferior or chloritous chalk (green sand), to which I consider 

 it may be associated. In one of these situations the mine- 

 ralogical characters entirely disappear, the geological posi- 

 tion is obscure, and the zoological characters alone remain. 



* From this it would appear that the upper or white chalk of M. Brong- 

 niart, is our flinty or upper chalk, with most probably our white chalk 

 without flints or lower chalk; that his middle chalk is our grey chalk, 

 and perhaps chalk marl, and that his inferior chalk or glaucoiae cray. 

 euse, is our green sand. (Trans.) 



