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Part I. Geometrical and Optical Properties.The goniometrical measurements of N. Story-Maskelyne 1 ) on crystals of tenorite (also called melaconite) found with cuprite and chrysocolla in a chloritic veinstone from Lostwithiel, Cornwall, satisfy the symmetry requirements of the monoclinic system, and the crystal habit illustrated in the drawing of Story-Maskelyne is clearly that of the holohedral class of the monoclinic system. Moreover, as A. Scacchi 2 ) has pointed out, the goniometrical measurements of J. Jenzsch 3 ) on artificial crystals of cupric oxide formed near a flue in a smelterhearth during roasting of copper matte with sodium chloride at Freiberg, yield calculated elements agreeing with those of Story-Maskelyne, and the crystals drawn by Jenzsch exhibited the same habit and twinlaw as those of Story-Maskelyne. Jenzsch interpreted his crystals as orthorhombic but his measurements do not satisfy orthorhombic symmetry requirements either qualitatively or quantitatively.Numerous inconsistencies and errors in detail occur in the papers of Story-Maskelyne, of Jenzsch, and of Scacchi. For this reason we have carried out geometrical and optical, as well as X-ray measurements, sufficient to establish our conclusions without the use of previous experimental results. At the same time our conclusions are in agreement with previously reported experimental facts. Our conclusions rest on optical, geometrical, and X-ray study of the thin natural tenorite crystals deposited on the lavas of Mount Vesuvius, on X-ray study of chemically pure artificial cupric oxide powder, and finally on geometrical and X-ray study of single faceted crystals of tenorite prepared for the purpose by recrystallization in molten sodium hydroxide. 1) Verh. Petersb. Min. Ges. 1 (1866) 147. 2) Contribuzioni mineralogiche per aervire alia storia dell'incendio vesuviano del mese di Aprile

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