tinstone. 441 planes; and again flatly acuminated with four planes, which are set on the obtuse edges of the first acumination *, fig. 253. _ The edges of the second acumination are sometimes truncated f- 7. Twin crystals of various descriptions ; but of these the most frequent is that formed by the junction of two crystals, of the variety No. 2. which is represented in fig. 254 The twin crystal here figured, is one of the most com mon forms of the species- The surface of the crystals is usually smooth, seldom more or less strongly streaked, and it is commonly splen dent. . i Internally it is only, shining and glistening, and the lustre is intermediate between resinous and adamantine, hut more inclining to the latter. ^ The fracture is coarse and small-grained uneven, incli ning to imperfect conchoidal; seldom imperfect foliated, and extremely seldom perfect foliated, and then it is high ly splendent. The fragments are indeterminately angular, and rather hlunt-edged. The massive varieties generally occur in coarse, small a nd fine'granular distinct concretions. It alternates from semi transparent to opaque i the darker coloured varieties are opaque, the lighter translu cent * Etaiu oxyde opposite, Iluuy. t Etain oxyde diatique, llauy. t The most complete account of the various crystallisation of tinstone we posses.-, is that by Mr William Phillips, in the second volume of the Transactions of the Geological Society of London. His memoir is accom panied with a series of beautiful plates, of which 1 could not avail myself 'n this work, because of their number and minutfenefs.