Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/362

 2. Some Remarks on the Denudation of the Oolites of the Bath District, with a Theory on the Denudation of Oolites generally. By W. Stephen Mitchell, Esq., M.A., LL.B., F.G.S., of Gonville and Caius College, Cambridge.

[Abridged.]

The theory commonly held as to the origin of the Oolite hills of this country is, that the strata of which they are composed were originally deposited in sheets continuous over large areas, that the valleys are entirely the result of denudation, which has left the intervening masses standing out as hills, and that it is generally possible to recognize on opposite sides of a valley at the different horizons the individual beds which were formerly continuous across that valley. The author believed that the following theory had equal claims to credence : —

That the sedimentary deposits of any particular Oolitic district were probably originally deposited in continuous beds, but that the limestones in many instances probably never extended beyond the areas they now occupy.

The object of the paper was to show that the Bath district was one of these instances; and the following were the chief points brought under the notice of the Society.

In the Bath district, while the beds of the Great Oolite limestone are for the most part approximately horizontal over the plateaux of the downs, they thin out at the edges of the hills. Mr. Sanders in his map of the district has marked the few Oolite hills which come within the map as everywhere dipping towards the valleys.

There is no evidence here of a washing away of the underlying Fuller's Earth, and a consequent drop of the limestone, as is suggested by Mr. Witchell to be the case in the Cotteswolds*. The Fuller's Earth might be scooped away for many feet under the limestone before the latter would move. When it did move, it would probably break off and slip or roll down the hill-slope. Here the thinning out is the same when other beds of the limestone underlie ; there is no displacement from the general mass. [It is just possible that they may be cases of false bedding, with the layers that were above them swept away ; but the author could not see by what agency this would be effected.]

To explain this thinning out, the probable origin of the beds was taken into consideration.

I. The limestones. — The commonly received view that the material of the Great and of the Inferior Oolite was accumulated by corals was supported by quotations from Jukes's ' Voyage of the Fly,' and Darwin's ' Voyage of the Beagle.' Recent coral-reefs are described as consisting of " rock fine-grained, only here and there exhibiting any organic structure." They show lamination and a jointed structure; and the bedding round the edges of an island dips towards the sea.


 * Proceedings of the Cotteswold Nat. Club, 1867.