290 PROCEEDINGS OF SECTION ©. 
origin. North of Lefroy, in Tasmania, some of our sub-. 
aerial Tertiary basalt is ophitic or coarsely intersertal—a 
typical dolerite. | Consequently, we are not called upon to 
accept submarine conditions for the eruptions of our diabase 
by reason of its structure alone. 
Although the structure of the rock throughout the Island 
is monotonously uniform, there are slight variations in two 
opposite directions. 
On the Brown’s River Road the coarse diabase contains 
stouter prisms of felspars, which in sections show correspond- 
ingly broad forms, but not so isometric as in true gabbros. 
Occasionally this variation is prolonged until quite gabbroid 
diabases are developed. The other direction in which it 
varies is towards the development of an intersertal and even 
porphyritic structure. This is illustrated by the quarry at 
the Railway Station, Hobart, where porphyritic stone may 
be picked out sometimes. Such structure may be explained 
by proximity to the margin of the intrusion. 
Mineralogical Constitution.—Our diabase is essentially a 
mixture of lime-soda felspar (labradorite) and augite, accom- 
panied by magnetite and ihmenite, and always a little 
apatite. Occasionally hypersthene occurs sporadically, 
more rarely biotite, and possibly olivine. Chlorite is always 
present in more or less quantity. Quartz occurs, often in 
granophyric intergrowth with felspar, which is probably 
orthoclase. This is seen in the diabase of some of the moun- 
tains round Hobart. (Mt. Faulkner, Organ Pipes on Mt. 
Wellington, Brown’s River Road, &c.) 
A rhombic pyroxene has been noticed in the rock at Both- 
well, Ross, Tiers west of Tunbridge, Mt. Direction, Killa- 
faddy, Devonport. The augite in the diabase dykes which 
traverse the granite at the Blue Tier appears to be largely 
uralitised. 
Mention may be made of the Bothwell Tertiary basaltic 
lava having been extruded through the Mesozoic diabase 
cap, as evidenced by included fragments of the latter de 
tected microscopically in a slide of the basalt. 
Chemically the rock is identical with basalc, and as any 
structural difference is due to geological occurrence, and not 
to differentiation of magma, it cannot be ranked among the 
dyke rocks, even though it be intrusive. 
The preceding remarks have reference to the diabase of 
Upper Mesozoic age, but there is a group of imperfectly- 
known rocks on the Magnet Range of much older age. 
These can at present only be generally mentioned as diabase 
and diabase porphyrite. Their geological horizon has not 
