12 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 66. 



a = 1.697, i3 = 1.710, 7 = 1.725, 2V medmm, r > v slight. The extinction 

 Z AC is 43°-48°. Sections on (010) are frequently encountered where, 

 as^shown in figure 1 , the basal parting is combined with twinning on 

 (100), giving a "herringbone" appearance. Frequently these give 

 uniform extinction in both individuals of the twin due to the extinc- 

 tion angle being exactly 45^^, X of one individual coinciding with Z 

 of the other, as is easily shown with a quartz wedge. In some sec- 

 tions the basal lamination and twinning are absent except in a line of 



pyroxenes across the section, 

 and a long prismatic pyroxene 

 which lies athwart the line may 

 have the polysynthetic twin- 

 ning where the line intersects 

 the crystal and not in the other 

 portions which may be clear. 

 Along these lines there is a con- 

 centration of iron ore and bio- 

 tite which are thought to be 

 late introductions as well as 

 secondary chlorite and other 

 alteration products which seem 

 to indicate the presence of mi- 

 nute fractures. 



Biotite is always present in 

 thin sections but is never con- 

 spicuous. Occasionally a large 

 clear-cut grain is seen, which 

 maybe an early crystallization, 

 but usually the mineral occurs 

 as small ragged grains grown 

 around the boundaries of py- 

 roxene or iron ore and more or 

 less associated with a yellowish green fine scaly serpentinous or chlo- 

 ritic alteration product. The biotite may in large part be a late reac- 

 tion product. It is mostly of the usual type with small axial angle, 

 biaxial negative character, and intense pleochroism in Ught and dark 

 red-brown shades, the absorption being, however, unusual in that it is 

 greatest in the direction perpendicular to the cleavage. Some crys- 

 tals are pleochroic in pale brown to almost colorless parallel to the 

 cleavage and violet-black to greenish black and almost opaque per- 

 pendicular to the cleavage. Minute ragged grains of biotite occur 

 mixed with grains of hornblende and opaque iron ore in altered 

 pyroxene individuals, the biotite here evidently being a secondary 

 product derived from alteration of the augite. 



Fta. 1. — Augite from diabase; sectiok on b(OlO) 



SHOWING TWINNING AND PARTING PARALLEL TO 0(100) 

 AND CLOSE FARTING PARALLEL TO C(OOl), ALSO SHOW- 

 ING EXTINCTION OF 45° PRODUCING SIMULTANEOUS 

 EXTINCTION IN BOTH HALVES OF THE TWIN. 



