Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/953

 Turning next to the south of the quarry, once again the red prisms are seen at an angle of 38°. This strange-looking rock passes downwards into a coarse greenish-looking bed, composed of fragments of limestone and partings of greenish-yellow clay (c) ; and this forms a " wadding " to a series of very excellent ornamental marbles, which form three principal beds with their partings ; these marbles are full of corals, the ramifications of which give them their beautiful variegated appearance.

Fig. 1. — Section of altered Clay-bed in Tideswell Dale.

a1. Toadstone with concretionary balls a2. "     very hard, dark green, and compact about 12 ft. a3.    "      amygdaloidal and vesicular b. Prismatic (clay ?),                         9 ft. c. Clay and Limestone, " wadding." d. Marble with Corals, facing south.

With regard to the red clay just described, the question which suggested itself to me was, whether it was a greater local development of that before mentioned as occurring in thin partings in the limestone near Litton tunnel. I am not aware of any other locality in the county where the clay is found altered as this is, and partaking of the columnar character, which must have been caused, I presume, by the heating effects of the overflow of the toadstone, and subsequent contraction under pressure. I have seen brick-clay that has, on a small scale, become beautifully prismatic after heating ; and we have instances of clays and other rocks being altered and rendered more or less perfectly columnar through proximity to eruptive rocks. I cannot do better, in conclusion, than quote a remark made by Mr. D. Forbes a short time since in reference to the igneous rocks of Staffordshire. He said that " the sedimentary rocks in contact with these were themselves frequently so altered as to present in themselves a columnar structure or jointing, in some