209 
A New Occurrence of Olivine Diabase . 
Levy’s or Pumpelly’s method of measuring the extinction angle in those 
sections which give symmetrical extinctions (zone of b), furnished a max¬ 
imum value of 28°, making it probable that the feldspar is labradorite. 
A tolerably fresh piece gave a specific gravity of 2.695. Some crystals 
show a decided cloudiness, which is found to be due in part at least to 
the formation of a colorless micaceous mineral. 
The augite, which is penetrated in all directions by plagioclase laths 
presents some interesting characters. The usual cleavage parallel to 
the prism is well marked with good partings parallel to one or both of 
the vertical pinacoids. In some basal sections all four of these are well 
developed. The color of the mineral is reddish to yellowish-brown, but 
with varying depth of tone, and sometimes grades into an olive near the 
periphery of the section. The dichroism of basal sections is most 
marked, the ray vibrating parallel to the plane of symmetry, being yel¬ 
low-brown with a slight tinge of red, and that vibrating in the perpen¬ 
dicular direction being distinctly red-brown. The monoclinic symmetry 
of the mineral is evinced by the inclined optic axis always obtained in 
sections which show nearly perpendicular cleavages, and by the high 
extinction angles (as high as 45°) in those sections which show but one 
cleavage. A brown or greenish brown hornblende is sometimes found 
as a partial peripheral zone about or inclosed within augite crystals, 
generally with approximately the same orientation as the augite. It 
appears to be original, its position being explained by parallel growth. 
The hornblende cleavage and the marked pleochroism serve to identify 
it. The pleochroism is c deep blue-green or brown, b deep green or 
brown, and a light green or light reddish brown. The absorption equa¬ 
tion is c = b > > a. In one of the two sections there is considerable 
green hornblende in good crystals, grouped about areas filled with more 
or less unresolvable material (probably both hornblende and chlorite 
and doubtless pseudomorphs after olivine crystals). 
Olivine is present in one of the sections in crystals which are identi¬ 
fied by their peculiar hexagonial outlines, colorless character, rough 
surface, parallel extinction, imperfect cleavage, and large optical angle 
in the plane perpendicular to the cleavage lines (base). In this section 
the mineral is in many cases remarkably fresh, showing along cracks, 
however, a small amount of a bluish green substance which is also to be 
found about crystals. There are also small, more or less irregular areas 
particularly abundant in the vicinity of ilmenite, which are in part red- 
brown and pleochronic and in part greenish. The brown parts are 
made out to be brown hornblende, and the green portions chlorite, in 
part of the variety known as delessite. The chlorite sometimes shows 
very marked pleochroism from intense orange to bluish green, and has 
slightly inclined extinction. It is clear from the disposition of the 
chlorite that it is the alteration product of the hornblende as well as of 
14—A. & L. 
