In the specimen in my possession, the mineral occurs filling seams in the guano, varying from ⅛ to ¼ of an inch in width. The matrix itself is of the variety known as rock guano. It possesses an oolitic structure and a brownish white color, interspersed with small spots of pure white. The mineral is in the form of small but very perfect and brilliant crystals with a cleavage in the direction of their greatest length, nearly equal to that of selenite; the laminæ, being also slightly flexible, as in the case of the latter species. Hardness, 2.25 (Moh’s scale). Specific gravity, 2.208, (mean of two determinations). Color, yellowish white. Transparent. Lustre, vitreous splendent, inclining to pearly on the cleavage faces. When heated in a closed tube before the blowpipe, it whitens and gives off water, at an incipient red heat. In the platina forceps, it fuses with intumescence, at about 2. of Von Kobbell’s scale, tinging the flame with the peculiar green characteristic of phosphoric acid. The button formed by fusion, crystallizes on cooling, showing numerous brilliant facets. Readily soluble, even in coarse crystals, in dilute nitric and hydrochloric acids. A qualitative analysis, revealed the presence of Lime, Phosphoric acid, and water, with barely discernable traces of Magnesia and Alumina. The quantity of mineral at my disposal was very small, scarcely exceeding one gramme in weight. In each of the following analyses, the water was determined in 0.2 grammes, the remaining 0.3 grammes being employed in the determination of the Lime and Phosphoric acid. The results were as follows: 1. 2. Lime 32.65 32.73 Phosphoric acid 41.50 41.32 Water 26.33 26.40 ------ ------ 100.48 100.45 These figures agree exactly with the composition of the neutral tri-basic phosphate of lime (2 CaO., H O, P O₅), with the addition of four equivalents of water of crystallization, (2 CaO, H O, P O₅ + 4 aq.) viz.: 2 CaO 56.26 = 32.59 P O₅ 71.36 = 41.34 H O 9.00 } = 26.07 4 Aq 36.00 } ------ ------ 172.62 100.00 In the polarizing microscope, the mineral shows a vivid succession of colors. A sample has been sent to Prof. J. D. Dana, who has kindly undertaken the study of its crystallographic characters, and I hope, in a short time, to be able to communicate the results of his investigations to the Academy. It is with very great pleasure, that I dedicate this species to Prof. Geo. J. Brush, of Yale College, to whose unwearied zeal and efficient labors, American Mineralogy stands so deeply indebted. REGULAR MEETING, SEPT. 19TH, 1864. President in the Chair.