ON PYROXEXIC COMPONENTS IN CERTAIN VOLCANIC ROCKS. 75 



easily obtained in a fresh state from the matrix. These crystals have 

 a stout prismatic habitus, "5 — 1 cm. in the direction of the vertical 

 axis, usually flattened along the macropinacoidal face. They are 

 somewhat brittle. The larger crystals are dark green in colour, 

 the smaller ones transmitting a pistachio-green colour and in 

 still smaller crystals a light olive-green colour. The observed 

 faces,* when referred to the axial system of vom Rathf are as follows : 



The predominating faces are a, b, and e ; the prism m truncating 

 the edge of a : b as narrow bands. The face i is usually small (fig. 1), 

 Thus the crystal is rather simple, being a stout prism with a very flat 

 Pyramid e, and tabular along the Macropinacoid a. In this respect it re- 

 sembles the crystal of Hypersthene (Amblystegite) from Laach volcanic 

 bombs, described by v. Path. (1. c). The faces of the Pyramids e, i, 

 are very dull and sometimes depressed a little, while the pinacoids and 

 the prism-faces are brighter, but usually owing to surface irregularities, 

 they do not give a very satisfactory reflection. The following 

 goniometric measurements were performed with a small reflecting 

 goniometer (Füss' Wollaston model). It may here suffice to show 

 an agreement with the corresponding angles of Hypersthene given 

 by v. Rath (1. c. p. 530):— 



* In the position adopted by Tscheruiak (e. g. Lehrbuch der Mineralogie, 2te. Aufl. p. 436) 

 the crystal is so placed that the acute prism-angle is turned forward, in oi'der to bring it into 

 analogy with monoclinic Pyroxene. In this position, the signatures and the symbols of the 

 faces here mentioned are respectively, b (010), a (100), m (100), e (122), i (121). Fig. 9, repre- 

 sents a crystal formed of the faces a, b, m, and i figured in this position. 



f Mineralogische Mittheiluugen — Ueber ein neues Mineral aus Laach. — Poggendorf. Ann. 

 d. Physk. u. Chem. Bd. CXXXVIII, 1869. p. 529. 



