The crystal of calcite found by Professor Tait presents very large dimensions for a specimen, with very simple, and, at the same time, very definite forms. This crystal shows the faces of the primitive rhombohedron of 105°, and is twinned with parallel axes. The two individuals which compose the crystal show the form of the cleavage rhombohedron R (1011); they are applied to each other with symmetrical development with reference to the base oR (0001), and present the appearance of a simple crystal, although formed of two distinct halves, of which the upper belongs to one crystal and the lower to the other, the two individuals being complementary to each other. Along the twinning plane may be noticed a series of very regular grooves, which indicate a repetition of the twinning following the base, It must be noticed that the six faces do not present the same physical characters– two of them, the primitive faces of tho crystal, are smooth; the other four, although having the same crystallographic sign, are faces of cleavage more brilliant than the others. They appear to show that the crystal, although found isolated by Professor Tait, was formerly attached. This is further demonstrated by the presence of irregular faces, which are not amenable to any mathematical law. These false faces may be seen on the superior and inferior portions of the crystal; they are granular, and without lustre, and cannot be confounded either with the crystal faces or with those of cleavage. They have been produced by the pressure exerted upon the crystal by the neighbouring crystals, which were developing at the same time. This consideration explains the anomalies which they show, when regarded from a geometrical point of view.