IGNEOUS ROCKS OF TASMANIA. 291 
been definitely determined yet, but it is apparently higher 
than the Siluriam and lower than the Permo-Carboniferous. 
Among these older diabases, allusion may be made to a 
remarkable nodular variolite which bounds the west or 
hanging-wall side of the websterite dyke containing the 
lode of the Magnet Mine. It varies into a vesicular diabase- 
porphyrite. Professor Rosenbusch has kindly determined 
this rock, as follows: —‘“‘ If a slide be made of the soft dark- 
green ground-mass (which is soft enough to be scratched 
with a knife), it can be seen to consist of a scaly aggregate, 
the scales of which can often be recognised as chlorite, with 
very weak double refraction, and optically positive: optic 
axial angle very small. Pleochroism weak, normal — green 
for rays vibrating parallel with the surface of the flake, 
yellowish-white for those vibrating perpendicular thereto. 
In it are lying colourless sections, variously bounded, but 
always with crystallographic contours, long, rectangular, and 
prismatic, also nearly quadratic, extinguishing sometimes 
straight, sometimes oblique. In convergent light these 
often show the emergence of a positive bisectrix of a not 
very large axial angle, sometimes the emergence of a nega 
tive bisectrix of a very large axial angle. In the first case, 
no structure is recognised; in the second, a more or less 
scaly or fibrous structure. Their refractive index differs 
very little from that of the main mass, and there are often 
seen lying in these apparent crystals green heaps of scales 
without any clear boundaries, but passing into the colour- 
less substance and having the same optic orientation. In 
the colourless sections there are also lying homogeneous and 
homoaxial pseudomorphoses of chlorite, poor in Fe (Leuch- 
tenbergite) after a pyroxene mineral, but I cannot say 
whether the latter was monoclinic or orthorhombic. Further, 
in the green mass, there are circular hollow spots (nearly 
always surrounded by cracks), which were no doubt 
originally amygdaloidal cavities, but are now filled with 
mixed ‘chloritic and quartz spherulites of irregular architec- 
ture. It is quartz (optically + and uniaxial), not chal- 
cedony (optically — and biaxial). Finally, in the ground- 
mass, are little aggregations of iron ore, which I have not 
examined more closely. They dissolve easily in HCl, which 
also strongly attacks the chlorite and leuchtenbergite. Now, 
if a slice be made through the nodules, which are much 
harder than the ground-mass, and sometimes cannot be 
scratched with the knife, here and there chloritic spots are 
seen, containing small sections of chalcedony amygdules. 
Inside the nodules is sometimes some ground-mass. More 
