32 Sketch of a Classijication of the European Rocks. 



be said to have devoted their exclusive attention to them. The 

 group is characterized by the first appearance, in the ascending se- 

 ries, of any abundance of the mammiferous animals, many genera of 

 which are novi^ extinct. 



Group 4. [Cretaceous) contains the rocks which in England and 

 the North of France are characterized by chalk in the upper part, 

 arid sands and sandstones in the lower. The term "cretaceous" is 

 perhaps an indifferent one ; for, possibly, the mineralogical character 

 of the upper portion whence the name is derived is local, that is, con- 

 fined to a particular portion of Europe, and may be represented 

 elsewhere by dark compact limestones or even sandstones. As how- 

 ever the geologists of the present day are perfectly agreed as to 

 what rock is meant when w^e speak of " the chalk," there seems no 

 objection to retain it for the present. The French geologists have 

 long considered the sands beneath the chalk, known as green-sands, 

 as belonging to the same formation with the chalk. That the fresh- 

 water character of the shells contained in the Wealden rocks is more 

 or less local, it seems but rational to infer ; for it cannot be imagined 

 that all the waters of the globe became suddenly fresh in order that 

 these rocks might be formed, and as suddenly salt again for the de- 

 position of the green-sands and chalk. Some French geologists 

 moreover consider that in France there is a marine equivalent of the 

 Wealden rocks. 



As far as our observations of fossil organic remains have yet ex- 

 tended, it would seem probable that the ammonites and belemnites 

 ceased to exist after the formation of this group ; for, as yet, their 

 remains have not been detected in Group 3. Should this, after a 

 greater extent of the world has been examined, be found generally 

 true, it will be a most valuable guide in determining the relative ages 

 of this and the previously noticed group in cases where the minera- 

 logical structure is of no avail. 



Group 5. (Oolitic) comprises the various members of the oolite or 

 Jura limestone formation, including lias. The term oolitic has been 

 retained upon the same principle as that of cretaceous : in point of 

 fact even in England and the Nordi of France the oolites, properly 

 so called, form but an insignificant part of the mass of rocks known 

 by the name of the oolite formation ; this character is also not con- 

 fined to the rocks in question, but is common to many others. In 

 the Alps and Italy the oolite formation is replaced by dark and com- 

 pact marble limestones, so that its mineralogical structure is of no val- 



