204 



THE GEOLOGIST. 



whilst our freestone belongs to the inferior oolite ; and this 

 may serve as an illustration of the oft-repeated fact, that 

 lithological structure is no proof of the comparative ages of 

 different formations, but it is to the fossils, and these alone, 

 that we must look for accurate data upon which to found 

 our arguments as to the relative ages of different rocks. 

 One family of shells, in particular, will serve to distinguish 

 the great from the inferior oolitic beds, which is the 

 Patella ; of these the great oolite contains about six species, 

 whilst the inferior oolite contains but one. 



Pecten vagaris, Trigonia clavellata, Cuculeea oblonga, 

 together with several small species of univalve shells, 

 amongst which are the Rostellaria and Melanea, also disthi- 

 guish the great oolite. The fossils peculiar to the freestone 

 ofour oolite are the following : — 



Plagiostoma punctata, 



„ cardiformis, 

 Terebratula fimbria (the most abundant sheD), 

 media, 



and two beautiful species of corals, the names of which I 

 cannot ascertain. 



4. At the top of the last named bed of freestone, a stra- 

 tum presents itself of a totally different character, as in it 

 we lose all traces of oolitic structure, the masses of stone 

 present more the appearance of chalk, being white and like 

 compact marl ; hence I have named it oolitic marl ; its frac- 

 ture is uneven and eccentric, and presents no appearance of 

 cleavage ; it is rich in fossil remains, although it contains 

 no great variety of species ; still, the fossils peculiar to it 

 are abundant, and well mark the 1)ed in whicli they are 

 found. The following is a list of those at present in my 

 collection : — 



