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University of California. 



[Vol. i. 



oblique extinction in the plane of the clinopinacoid, and the emer- 

 gence of an optic axis in the orthopinacoidal section. The polari- 

 zation colors are quite brilliant. It occurs partly in idiomorphic 

 crystals and partly allotriomorphic with reference to the feldspar. 

 It is very slightly pleochroic with pale brown to greenish tints. 

 Numerous basal sections exhibit an octagonal form and very dis- 

 tinct prismatic cleavage. The idiomorphic individuals are elon- 

 gated in the plane of the orthopinacoid to long tabular forms. Twin- 

 ning is very common; in some individuals there are but two simple 

 twins, in others a thin lamellar plate separates the two broad outer 

 ones. 



In addition to the twinning another divisional plane appears, 

 which is so closely allied to it that, not until after close study and 

 repeated examination, was it shown to be different. Twinning in 

 augite takes place in the plane of the orthopinacoid, consequently 

 sections cut in the plane of the clinopinacoid will extinguish sym- 

 metrically on either side of the trace of the composition plane, and, 

 if the extinction angle is 45 °, both will become dark at the same 

 time. Twins cut perpendicularly to the prism will also extinguish 

 simultaneously and parallel to the composition plane. This other 

 parting in the augite of the Cuyamas dike takes place in the same 

 plane as the twinning, and closely resembles it, but is distinguished 

 by the fact that, in whatever position the section is cut, the two 

 halves of the crystal extinguish simultaneously. Numerous indi- 

 viduals were examined which were cut in the clinopinacoid, or 

 approximated it, and, with very few exceptions (twinned indi- 

 viduals), all extinguished simultaneously. The extinction angle 

 was not 45 , and, consequently, as that is the only angle at which 

 twins cut in that section could extinguish simultaneously, the 

 phenomenon seen is not that of ordinary twinning. This plane, 

 the trace of which is seen in the clinopinacoid and basal sections, 

 is a very perfect one, geometrically considered. (Plate 15, Fig. 3.) 

 It is distinguished by its regularity from ordinary cleavage, and is 

 often so fine that it is inconspicuous. In some instances the parting 

 does not extend to the borders of the crystal, and still more rarely 

 there was observed a short parallel parting. A close examination 

 of this plane with high powers showed that it was not possible to 



