THE VICTORIAN NATURALIST. 69 
The term Tachylite has been used by Zirkel for a glassy modifica- 
tion of Basalt.* Rosenbusch restricts it to those “more or less 
easily decomposable basie hyaline rocixs which would have produced 
a Basalt had erystalline consolidation taken place.” More lately 
he has suggested that it will be well to use the word Vitrophyr for 
all voleanic rocks, which are mainly or wholly composed of glassy 
material. There would therefore be a Basalt-Vitrophyr and the 
name Tachylite might be added as indicating that sub-division to 
which the rocks, which are readily decomposed by Hydrochloric Acid, 
belong. 
The sample which I have examined has the composition and 
structure of a hyaline example of Basalt, and according to the rules 
indicated by Rosenbusch, it can be deseribed as a Basalt-Vitrophyr 
of the Tachylite sub-division. 
2. Crystalline rock. This sample formed part of a rock fragment 
embedded in the Tachylite. I prepared a thin slice which | found 
to have many resemblances to that last described. The ground 
mass 1s composed of much dark-colored glass, which is made still 
more opaque by a great amount of black dust-like particles 
(magnetite?) In this are many lath-shaped compound triclinic 
felspars and some irregular grains and clusters of grains of Olivine, 
and a few rare grains which may perhaps be a light-colored Augite. 
The vescicles of the rock are filled in with carbonates. Optical 
measurements which I obtained in some of the felspars gaye me 
results which, although not very satisfactory, allow it to be said that 
in the zone oP (001) — 0 Pa (100) the inclination of the angles 
of obscuration lie between 7° 30! and 33° 80!. One section which 
was near to the Brachypinacoid gave an angle of 39° 30!. 
Such angles as these suggest a basic felspar Labradorite or 
Bytownite series, but the angles taken as a whole do not agree 
with those of either one or the other, as given for instance in the 
researches of Schuster on the optical properties of the Plagioclastie 
felspars. 
This rock may be considered as being a fine grained somewhat 
vesicular Basalt, with a large amount of glass in its ground mass. 
3. Cement. This sample was taken from part of the “ cement? 
already spoken of. I found in examining a thin slice that it is 
composed of quartz sand, with a few larger fragments of what seems 
like vein quartz, and some black flecks which may be probably 
carbonaceous material. It has evidently been part of the river 
drift upon which it was found resting, and may have been altered to 
its present condition, at the time when the flow of Basaltic lava 
sealed up the course of the ancient river. 
To sum up the preceding statements so far as relates to the 
Weeanie rock, through which the shaft of the Tanjil Deep Lead 
