singularly resembles some of the branched species of Nullipora. Under the blowpipe, it behaves like the specimens from Ascension; but it is less hard and glossy, and the surface has not the shrunk appearance. [6] In my “Journal” I have described this substance; I then believed that it was an impure phosphate of lime. Chapter III ASCENSION. Basaltic lavas.—Numerous craters truncated on the same side.—Singular structure of volcanic bombs.—Aeriform explosions.—Ejected granitic fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of formation.—Concretions in pumiceous tuff.—Calcareous deposits and frondescent incrustations on the coast.—Remarkable laminated beds, alternating with, and passing into, obsidian.—Origin of obsidian.—Lamination of volcanic rocks. This island is situated in the Atlantic Ocean, in lat. 8° S., long. 14° W. It has the form of an irregular triangle (see map below), each side being about six miles in length. Its highest point is 2,870 feet[1] above the level of the sea. The whole is volcanic, and, from the absence of proofs to the contrary, I believe of subaerial origin. The fundamental rock is everywhere of a pale colour, generally compact, and of a feldspathic nature. In the S.E. portion of the island, where the highest land is situated, well characterised trachyte, and other congenerous rocks of that varying here and there a hill or single point of rock (one of which near the sea-coast, north of the Fort, is only two or three yards across) of the trachyte still remaining exposed. [1] _Geographical Journal_, vol. v, p. 243. Illustration: Island of Ascension PLATE IV.