MOUNT RORAIMA, in British Guiana, was ascended for the third time on October 14 last, by Mr. F. Dressel, an English orchid collector. The first ascent, it will be remembered, was by Mr. Im Thurn, in December 1884. The second was in November 1886, by Mr. Cremer, also an orchid collector. Mr. Im Thurn's ascent took place in the beginning of the wet season, when everything was saturated with moisture. Mr. Dressel ascended after continuous dry weather, and found the upper surface comparatively dry, the elevated portions most markedly so; while large areas of the sward-like levels were perfectly desiccated. The water in the various channels was very shallow, and the deep basins or depressions contained but very small quantities, though in no case was any found to be quite dry. Frequently the surface of the water in these shallow basins was more or less covered with a green, apparently a Confervoid, layer. In the pools at the bottom of these wide basins, Mr. Dressel found a considerable quantity of quartz, in the form both of separate crystals, and of aggregated masses, of various and often of large sizes. The presence of such quartz in such positions and under such conditions, Timehri points out, is an extremely interesting fact, though our want of knowledge of the petrographic character of the formation of the top of Roraima, beyond the fact of its being sandstone, renders it barren, and one hardly justifying speculation. It will be remembered that on the first ascent no animal life was noticed during the short time spent on the top; and this necessarily denoted the likelihood of the absence or great rarity of birds and insects. During the two or three hours spent on the top by Mr. Dressel, no birds were seen; but a few specimens of butterflies, all of one kind, of a dark brown and nearly black colour, were observed, and two of them were caught, though one alone was sufficiently preserved to show much of its structure. In the shallow basins a few forms of a small black toad with a yellow spot on the throat was also seen, and one was caught, but was accidentally left on the top. A third animal form was found in the moist earth attached to some plants which had been pulled up; from Mr. Dressel's description it is conjectured by Timehri to be a Millipede, allied to Julus. It is probable enough that a stay of a day or two on the top would well repay the naturalist; and Mr. Dressel thinks it would not be difficult to arrange for such a stay. The fantastic shape into which the sandstone has been fashioned, and the weirdness of the scenes which have been so graphically described by Mr. Im Thurn, affected Mr. Dressel in a similar manner. He mentions that the surface of the rocks present very closely the appearance of granite, owing to weathering; and at first he thought some mistake had been made in describing the formation as sandstone, until he moved away a small rock from its setting, when its real nature was revealed.