68 THE VICTORIAN, NATURALIST. 
fill in cavities or cluster along the fissures of the rockin botryoidal 
or mammullated groups. 
The included fragments of crystalline rock vary from an almost 
compact to a vesicular texture, and in color from dark lead tint to 
almost black. 
I examined three samples. The unaltered glass, one of the in- 
cluded fragments of rock, and the “cement.” ‘he following are the 
results of the examination :— 
1. Volcanic glass,—It has a black color and a glassy lustre, with 
a peculiar wrinkled appearance on a fracture surface. A thin slice 
examined under the microscope proved to be in greater part a yellow 
isotropic glass, containing but few products of devitrification. In it 
are (@) very numerous, small, lath-like crystals of a triclinie felspar. 
Many of the crystals are imperfectly terminated, or the component 
plates are not of the same length. I could not obtain any reliable 
optical measurements, but the inclination of the plane of vibration is 
large, and suggests labradorite. (6.) Colorless angular grains or 
clusters of grains, which polarize in bright tints of green and red. 
Some few bear resemblance to colorless pyroxene, but I believe that 
all are olivine. Two groups showed traces of a rhombic prism, with — 
angles very near those of olivine. The roughened surface appear- 
ance of the grains, the occasional occurrence of minute included 
octahedra of magnetite, and the decomposition of similar grains im: 
another slice by hydrochloric acid confirm the conclusion. (c.) 
Magnetite, or perhaps titaniferous magnetite, occurs rarely. (d.) 
Finally, there are a few instances of alteration products which are 
most likely carbonate of iron, lime, &e. 
In order to learn more as to the character of this rock, I made a 
quantitative chemical analysis, of which the subjoined are the results :— 
P: O; wed Fe O 732 Naz O 3°30 
Ti O5 tr. Mn O tik Hig tO ‘79 
Si Og ‘HIS Ca O 8-74 
Al, O3 18:03 Me O 560 100-18 
Moisture -70 Sp. Sts: | 26 
The microscopical examination shows that this rock is essentially 
glassy, and that it includes porphyritically crystals of olivine, plagio- 
clase, and magnetite. These data, as also the quantitative analysis, 
agree best with a rock of the composition of tachylite, with which 
also the specific gravity falls in well.* 
*Zirkel, Lehrbuch der Petrographie, p 304. I refer to the analysis by 
Gmelin of the Tachylite of Bobenhausen. 
