BY GRAHAM OFFICER, B.SC. 151 
broad, undulating valley known as the Vale of Cuvier. This 
valley runs in a 8.E and N.W. direction, and is bounded on 
the one side by the Olympus Range and on the other by 
Mount Hugel. 
Having thus briefly sketched the main physical features of 
the lake and its surroundings, we will be in a better position 
to consider the geology. 
According to the map in Mr. R. M. Johnston’s excellent 
work “ The Geology of Tasmania,” and I believe this is the 
most recent, the southern and western shores of Lake St. 
Clair are represented as sedimentary rocks of upper palaozoic, 
or possibly mesozoic, age. I shall speak of them as car- 
boniferous, provisionally, all to the west of this is put down 
as greenstone (diabase). A small patch of basalt is represented 
as occurring about the S.E. extremity of the lake. The 
carboniferous rocks are marked as abutting on the older 
paleozoic, along a line running up the northern side of the 
Vale of Cuvier. ; 
The most important point at present in the geology of this 
district is the relation of the greenstone to the carboniferous 
rocks. The earlier geologists, notably Gould, Strzlecki, and 
Tenison- Wood, were of opinion that the greenstone was post- 
‘ecarboniferous. However, this has been called in question, 
and Mr. Johuston, though from what he says there would 
appear to be two greenstones of distinct ages, thinks that “the 
massive greenstones occupying the more elevated mountain 
ranges, as well as the greater part of the dividing ranges 
within the system, have all been erupted prior to the deposit, 
even of the lower members of the carboniferous system, and 
that only certain minor ridges, like that at Spring Hill, 
represent diabasic greenstones of a later date.” . . . “The 
great mland greenstone plateau of the lake country,” 
according to Mr. Johnston, “probably formed an elevated 
island mass of considerable extent” (in the carboniferous 
sea).* 
It may be said that Mount Olympus affords the key to the 
geology of the district. As already remarked, the Olympus 
Range skirts the western shore of Lake St. Clair, terminating 
at the lower end of the Vale of Cuvier. One of the best views 
of the mountain is obtained from about half way, or a little 
more up the lake. Here the crest of the mountain, rising to 
2,300 feet above the water, consists of massive columnar 
greenstone, the columns rising vertically for several hundred 
feet, their bases being concealed beneath the ruins of their 
fallen comrades. Below this greenstone crest appear hori- 
zontal beds of sandstone, extending to the shore of the lake 
below, and forming apparently the base of the mountain, 
* “Geology of Tasmania,” by R. M. Johnston, F L.S., pp. 102, 8. 
