Zoological Characters in Geology. 241 



lived on the spot, and at the period when the species to 

 which they belong were characteristic. 



I am aware of all these difficulties : I am on my guard 

 against these causes of deception, which introduce the un- 

 certainty and difficulty which we meet with in other sci- 

 eoces, into geology, and which require continued attention 

 and labour from the geologist, so as with discernment to 

 choose the species from which he should derive his charac- 

 ters, and attach their true value to them. 



I have then examined with all the attention that circura- 

 stancea have permitted, the influence of these different 

 causes on the structure of the chalk rocks, of which I am 

 abovtt to speak. 



These rocks are so singular that I considered it neces- 

 sary to precede their description by the general consider- 

 ations above brought forward, and to prepare naturalists for 

 recognising as chalk a hard and black rock which occurs at 

 a height of more than 2000 metres [6560 feet] on the sum- 

 mit of a mountain so difficult of access at certain periods, 

 that I have been unable to attain the point where it is 

 found. 



But before I proceed to the determination of this singular 

 chalk, I shall examine some others, whose differences being 

 less singular, will lead us less suddenly to that with which I 

 shall terminate this notice. 



§ II. Chalk of Rouen ^ of Havre, and of the Coast from 

 Honfleur to Dives. 



A hill named St. Catherine is observed near Rouen, at 

 .the eastern entrance to the town; this steep hill presents 

 the union of the upper white chalk with the craie tufau and 

 the inferior chlorite chalk (green sand), and leaves no doubt 

 , as to the identity of formation of these two rocks ; but these 

 last contain a great quantity of organic remaini, which differ 

 from those found in the white chalk. This union of cir- 

 cumstances is very favourable for observation, and we pre- 

 sent it the first, because it affords us the means of referring 



