Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/180

136 in the Trias on the flanks of the Mendips. All observers, however, agree that the rock is of a remarkably peculiar and almost unique character, Mr. Symonds calling it "a most distinguishable rock." To obviate confusion, I use a term which does not involve any theoretical views, and which was first applied to it by Dr. Gordon of Birnie, who has done so much for the elucidation of the geology of this difficult district, both by his own observations and by constantly placing his great local knowledge at the service of other investigators.

The mass of the Stotfield rock is composed principally of calcareous and siliceous materials. The former is usually a hard, compact, impure limestone, of a cream-colour, which in places becomes crystallized and exhibits fine examples of calc-spar, with fibrous and radiated carbonate of lime. The latter forms nodules and cavernous masses imbedded in the former, and consists of compact cherty or flinty material, occasionally exhibiting the banded structure of jaspers and agates, and containing drusy cavities incrusted with crystals of quartz or mammillated coatings of chalcedony. With these principal materials there is often associated an amorphous argillaceous mineral, of a greenish colour; while beautifully crystallized galena, pyrites, and blende are sometimes disseminated through the mass as accidental ingredients.

The total thickness of this rock is unknown, its upper portion being always denuded away; but it is said to have been dug to the depth of 30 feet. When exposed to weathering action, the calcareous portion of the rock is removed, and the hard indestructible masses of cherty material remain. Where the rock is covered with Boulder-clay, pits were formerly dug into it; and the most purely calcareous masses being selected, they were burned into lime; but the tendency of the material, when the siliceous portions were not rigidly excluded, to fuse into solid masses in the kiln, and the superiority of the Silurian limestones of Banffshire, have led to the almost total abandonment of these old pits. Several attempts have been made at Stotfield to work the galena, which is associated with this rock there, as also at Inverugie and other points; but, owing to the fact of the metallic ores being disseminated through the mass and not collected into veins, these attempts have proved futile.

The origin of this peculiar rock, which is altogether destitute of any trace of organic remains, is a very interesting problem. That its formation must be referred to purely chemical agencies is, I think, in the highest degree probable; but into this question I do not propose to enter, it being sufficient for my present purpose to point out the highly peculiar, if not unique, character of the rock, and the consequent facility with which it can be identified.

B. "The Reptiliferous Sandstone." — This lower division of the formation consists of beds of sandstone, sometimes slightly calcareous, and usually of a pale colour inclining to yellow; it has often grains