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I 56? CALC GENUS* yet parallely on the broader lateral planes, fo that it has a rhomboidal afpedj , when very fliort it paffes into the rhomb. 2. The fame prifm accuminated at both extre mities by four planes, which are fet on the lateral edges that bound the two larger planes. 3. Twin cryltals which are formed by the incor- poration of two of the preceding cryltals» in the direction of their breadth, in fuch a manner that the united fummits at one ex tremity form a re-entering angle, but & the other a four-planed acumination. 4. Sometimes the prifms, 1. 2. are terminal by fpherical convex planes. If the prih 11 i difappears, and thefe planes come together? j a fpherical convex lens is formed. 1 he lens is often columnarly aggregated. I he broader lateral planes are fmooth, the other® are longitudinally {freaked and Ihining. r Ihe pl^ eS of the lens are dull and rough. Cryftals are feldom large, generally middle fized- | ^Infernally its luftre is fliining and fplendent, and * s pearly. Fradure is perfedly foliated, molfly ftraig^ often curved, with one perfed and two impe^ cleavages; the two latter interfed the former obHq ue ' angularly, and they are interfeded redangularly .the firft or perfed cleavage, and this arrange' 1 ^ 11 gives rife to rhomboidal fragments.
