Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/360

 1. They present very distinct mineralogical characters, separating them alike from the overlying Neocomian and the subjacent Wealden proper.

2. They are of considerable thickness, attaining a maximum of 230 feet.

3. They present evidence of having been deposited under conditions differing alike from those of the marine Neocomian above and the purely freshwater Wealden below.

4. They yield a considerable marine fauna (between 30 and 40 species being already known), which is remarkably distinct and well characterized.

5. They are the undoubted representatives of a formation which in Spain attains to a vast thickness, and which, alike from its marked palaeontological characters and its great economic value, is of great importance.

6. Their relations to the Wealden and Neocomian are precisely analogous to those of the Purbeck formation to the Oolite and Wealden ; and they are therefore equally deserving with it of a distinctive title.

At the same time I have endeavoured to show that these beds may be regarded indifferently either as the highest member of the Wealden in our classification of the series of terrestrial strata, or as a portion of the Neocomian in our grouping of the marine series. The application to them of a distinctive name is therefore, although a necessary, perhaps only a provisional expedient ; but the same is to a greater or less extent true of most of our geological terms.

Discussion.

The President remarked that the limited amount of freshwater formations in this country was an obstacle to their correlation, and stated that Constant Prevost had endeavoured to correlate the Secondary freshwater and marine formations.

Mr. Godwin-Austen remarked upon the thinning out of the Lower Greensand, especially in France, upon the imperfection of our knowledge of the great Cretaceous formation, and upon the probability of the intercalation of freshwater conditions in the Lower Greensand. The formation at Punfield seemed to present an intercalation of marine between purely freshwater conditions. He indicated how a slight change of level might have intercalated marine conditions in the Wealden. The deposition of the White Chalk and Oolite occupied enormous periods (in both cases purely marine), during which the northern hemisphere was a great northern ocean ; and as the distribution of land and water was due to the operation of great cosmical laws, the duration of terrestrial and of the intermediate freshwater conditions was probably of equal length.

Mr. Etheridge observed that out of sixty or seventy species

employed the word in this paper, the term " system " being applied to the greater groups of strata.