156 THE GEOLOGY OF THE LAKE ST. CLAIR DISTRICT. 
map represents the rocks as entirely greenstone, but the road 
crosses several outcrops of sandstone, one being about a mile 
across. There are no good sections exposed along the track, 
go that the relations could not be clearly seen. , 
I think I have now shown good reasons for believing that 
the greenstone of the lake country is of subsequent date to the 
carboniferous (?) sandstones. Moreover it seems probable that 
these sandstones had been elevated and carved into all the 
varied features of a land surface when the floods of lava, now 
represented by the greenstone, overwhelmed the country. Since 
then enormous denudation has taken place, accompanied 
probably by considerable dislocations and displacements, and. 
the greenstone crests of Olympus, Byron, Ida, and all the 
other mountains of similar structure (in this district at least) 
are merely outliers, the remnants of a once vast and con- 
tinuous sheet of lava. The evidence of this enormous 
denudatior is about the most striking feature that catches the 
geologist’s eye when he ascends Mount Olympus for instance, 
giving him some idea of the magnitude of those forces which, 
though apparently trivial in themselves, are yet capable of 
producing such grand and imposing effects. 
The origin of the lakes of the great central plateau is a 
question which affords ample scope to any geologist who will 
undertake their investigation. 
Lake St. Clair was, I believe, first supposed to be a crater 
lake, but of this there isno evidence. Mr. Gould explained 
it on the theory of a flow of basalt damming up the lower 
end of the valley in which the lake les. However, I am 
much inclined to doubt the existence of this basalt. From a 
mere casual inspection, the rocks about this end of the lake 
seemed to us to be the ordinary greenstone. We had intended 
to make a special expedition from our camp to settle this 
point, but were prevented by bad weather at the last. 
The eastern third of the southern shore of the lake is 
bounded by a bank of sand which is covered sparsely with 
timber and in places is honey-combed by wombat-holes. The 
lake is very shallow in this locality, and knobs of greenstone 
can be seen projecting above the water at a considerable 
distance out from the shore. The upper end of the lake 
appears to be shallow as well as the lower. The ridge of sand 
just mentioned separates the lake from an extensive button- 
grass flat, which extends eastwards till it merges into the 
swamps that mark the entrance to the Derwent. It seems 
pretty certain that the waters of the lake once covered this 
flat. As Tasmania is undergoing a movement of upheaval 
the rivers must, geologically speaking, be rapidly lowering 
their channels. ‘Che course of the Derwent affords ample 
evidence of this. Thus the level of the Lake St. Clair waters 
must be gradually being reduced. At one place under 
