Mr. Mallet's Paper on Prismatic Basalt

Abstract

IN a paper published in the Geological Magazine for September last, and entitled “Note on Mr. R. Mallet on the Prismatic Structure of Basalt,” by Mr. Scrope, that author conceives he has found a refutation of the explanation which I have given of the production of the transverse joints in prismatic basalt in my paper on the subject published in the Philosophical Magazine for August and September 1875, and Proc. R. S. 158, 1875. For this he appeals to the features presented by the transverse joints in a group of three prisms stated to be from the Giant's Causeway, existing in the hall of the Geological Society of London. In one of these three prisms it is stated that the convex surfaces of the two top joints point in opposite directions, so that the upper articulation “is found to be biconcave in the fashion of a double concave lens. In another of the prisms the convex surfaces of the joints point downwards, while in the third the convex surfaces point upwards.”