DIABASE OF TASMANIA, 261 
dykes and masses of diabase showing at intervals. Through 
these sediments the Ouse has excavated a deep canon on the 
west of which they rise to an eievation of nearly 3000 feet 
above sea-level, often showing signs of alteration where a 
natural section is exposed, especially in some patches of 
sandstone north of Bashan Plains. On some small islands 
in Lake Echo there is a grey vesicular basalt, a more com- 
pact variety showing itself a little to the north, the typical 
clivine basalt occupying a considerable area about the Ouse, 
to the south-west of the Great Lake. Proceeding westerly, 
north of Lake Echo, we come upon patches of massive 
diabase, the greater part of the country being extensively 
covered with the talus derived from it. The basalt just 
mentioned, which is almost certainly of Tertiary age, is 
interesting as showing volcanic action in shallow valleys 
formed at this high elevation by the process of erosion, 
which may be assumed to have also removed the sediments 
once overlying the diabase of the higher mountains. Con- 
tinuing in a direction north of west, this route crosses a 
patch of flat-bedded hard grey mudstone at an altitude of 
over 3000 feet, with the diabase showing itself in ridges, 
and covering the surface in a broken-up condition, until 
the left bank of the River Serpentine is reached, near which 
there is an outcrop of the old micaceous schist of the 
Western Country. The Serpentine here cuts its way through 
the same Permo-Carboniferous beds that were seen on the 
right bank of the Ouse, and some distance to the north-east, 
near a branch of the Little Pine River, almost in the centre 
of Tasmania, there is a patch of the coal-measures with at- 
least one seam of coal. A continuation of this line west- 
ward to Lake St. Clair would probably show other similar 
evidence. The country south of it is occupied for the most 
part by basaltic plains and greenstone ridges. Mt. Olympus 
(diabase-capped), west of the lake, is flanked in the usual 
way by sedimentary rocks, and beyond it again is another 
remnant of the coal-measures resting on Permo-Carbon- 
iferous sandstone and marine beds, with plenty of evidence 
of alteration in the neighbourhood of the diabase. 
It should be understood, in explanation of the absence of 
detail in many parts of this paper, that the geological con- 
ditions described were, for the most part, observed by the 
author during very brief intermissions of official duty, or on 
forced marches across difficult country, when there was no 
time available for following up points of interest noted on 
either side of the route. 
Taking the subject as a whole, the conclusions may be 
triefly summarised : — . 
