Limestone. 489 On the furface of the cryftallized varieties (we muft except grey and red), a play of colours is to be obferved. The prifmatic varieties are ufually yel low, and of all the colours the yellowilh and greenilh are the moll charafteriftic. Befides maflive and difleminated, it occurs alfo in crults, ftala&itical, in globular and amygdaloidal fliaped pieces, in drufes, and kidney lhaped ; but it occurs ntoft frequently cryftallized, and prefents a great variety of cryftallizations. It pofleffes three fundamental figures to which all its other cryftallizations can be referred, thele are the 1. Six-fided pyramid. 2. Six-fided prifm, and 3. Three-fided pyramid. 1. Six fided Pyramid. When perfect it is acute, and three alternate la teral edges are more obtufe than the others. It oc curs, 1. Simple, and this is either ereft or inverted. The inverted has three cylindrical concave, and three inwardly bent lateral planes, and on the upper extremity it is flatly acuminated by three planes which are fet on the cylin- drically concave lateral planes. a. Double, where the lateral planes of the one are obliquely fet on the lateral planes of the other, in fuch a manner that the edges of the com mon