Geol.— Vol. I.] SMITH— SANTA CATALINA ISLAND. 39 



Where the original form is indicated it is very similar to that 

 of olivine. The form is best shown in the rock from near 

 the isthmus. That rock, however, is very soft and much 

 altered, and no satisfactory microscopic sections of the 

 mineral were obtained, except for the determination of the 

 outline. Several small but good crystals were made out in 

 the rock, with a lens. As seen thus, the general form tallies 

 with that described by Prof. Lawson.^ The mineral here 

 is deep brownish red in color, with a pronounced cleavage, 

 the cleavage surfaces presenting a somewhat metallic luster. 

 The central portion of the crystals is usually dark green. 

 The forms which the slides present are of two types, both 

 hexagonal, one with a pronounced cleavage, the other with- 

 out a cleavage but with a distinct fibration at right angles to 

 the direction of elongation. The cleavage subtends an 

 angle, two measurements of which gave 131° and 133°, 

 respectively. Referring the mineral to the same system of 

 axes to which Prof. Lawson has referred it, a is perpendic- 

 ular to the cleavage, b in the cleavage and parallel to the 

 fibration, c in the plane of cleavage and perpendicular to b. 



The cleavage is well shown by a series of parallel and 

 narrow, open seams, to which the extinction is in all cases 

 parallel. In sections in which the cleavage is wanting the 

 extinction is always parallel to the longer direction of the 

 section and to the fibration. The latter sections show a 

 fair biaxial interference figure. The mineral is therefore 

 orthorhombic. The emerg^ence of the acute bisectrix is 

 perpendicular to the plane of cleavage and \>--=b. The 

 optical character of the mineral, as determined by means of 

 the quartz wedge, is negative, a is therefore the acute bisec- 

 trix, and =a; z=c. 



The color of the mineral in thin section is very variable, 

 especially in different parts of one and the same section, 

 ranging from a golden brown to a clear though not bright 

 yellow, with occasional dull greenish areas in or near the 

 center. The deeper colors are usually marginal or along 



'^Loc. cit. 



