NO. 1882 pykite; carrying gold and galena — pocue 



481 



faces at right angles to these may all be equally inclined to the sur- 

 face of the pyrite. The mineral with this arrangement has a rhom- 

 bohedral appearance (imperfectly shown in the upper central por- 

 tion of figure 2, plate Lix), but its true nature is revealed by expos- 

 ing the cleavages, which are parallel to the external planes. At 

 times, as is shown in plate Lix_, figure i, irregular branching forms, 

 suggestive of fantastic figures, result from this orientation. 



The preceding conclusions were arrived at by a study of dozens 

 of cleavage fragments under the microscope and an examination 

 with hand lens of the galena in place, the prominent cubic cleavage 

 in all cases serving as a means of orientation. Measurements of the 

 cleavage by the microscope gave 903^°, 89^/2°, 90°, 89°. Several 

 fragments showed secondary octahedral cleavage, which is rare for 

 galena. One example is pictured in figure 83, in which a corner of 



Fig. 85. — Greatly enlarged cleavage surfaces of galena, showing 

 visible by incident sunlight under the microscope. 



groovmgs 



a cleavage cube is broken across by a series of smaller cubes, with 

 their corners, in turn, truncated by minute triangular octahedral 

 faces. 



An examination of cleavage surfaces under a high magnifying 

 power, illuminated by incident sunlight, reveals a complicated series 

 of striations and groovings. The striations are exceedingly minute, 

 visible as fine hair-lines only under the most favorable conditions 

 of reflection. There are two sets at right angles to each other and 

 parallel to the edges of the square cleavage fragments, which may 

 represent incipient cleavages ; a third set, less distinct, is sometimes 

 present, cutting the cubic cleavage and striations diagonally. The 

 last holds positions identical with the trace of octahedral planes. 

 More prominent than the striations, and much broader compara- 

 tively, are the members of a complicated series of sinuous grooves. 

 Figure 85 is a free-hand sketch of the pattern made by these. It 

 is seen that the groovings are predominantly parallel and diagonal 



