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288 PROCEEDINGS OF SECTION C. 
an occurrence there where it is strictly effusive, accompanied 
by tuffs and ejected blocks, and without any contact 
phenomena. He goes on to say thac the most crystalline 
varieties collected in these lava-flows are scarcely different 
from the normal type of the intrusive diabases. 
Rosenbusch admits that fresh diabases are indisctinguish- 
able from dolerite and intersertal basalt. Of this we have 
a striking illustration in the structure of the doleritic basalt 
north of Lefroy. The rock evidently formed part of a 
thick lava-flow, and must be regarded from its structure as 
olivine dolerite. Strictly speaking, the structure of an 
igneous rock is governed, not by its geological occurrence, 
but by the physical conditions of its consolidation. We are, 
however, often in a position to deduce either of these from 
the structure. 
Those who contend thac the heavy diabase caps which rest 
on the summits of our mountains are the lower parts of 
sub-aerial lava-flows must show a slaggy contact with the 
underlying sedimentary strata. This has not been shown 
yet. In some instances ic is difficult to avoid the inference 
that the eruptive rock does overlie the Mesozoic sandstone, 
as, e.g., at the Douglas and Denison rivers, where the 
streams have cut down into the diabase-crowned range, 
carving their channel through the overlying igneous sheet 
into the sandstone below. But here, again, the actual con- 
tacc is hidden by a heavy overburden of fallen blocks of 
columnar diabase. The very profusion and size of these 
prostrate columns lying on the sandstone bedrock lead me 
to infer that they have not been transported from furcher 
up the slope, but are simply the stationary remains of the 
former extension of the igneous cap. If, however, this 
igneous cap was not a sub-aerial flow, it must be explained 
as a lateral thrust from a hypabyssal intrusive mass, 2.¢., a 
sill or a laccolitic extension. For aught we know, the cap 
itself may in its central parts be a core. 
The bore at the Cascades, on the flanks of Mt. Wellington, 
passed from 509 to 519 feet through the Permo-Car- 
boniferous marine mudstones, and then struck the under- 
lying diabase, into which it penetrated 120 feet before 
boring was suspended. This may mean either of two 
things: (1) The underlying diabase is part of a mighty 
core which has intruded into the sedimentary strata, like a 
laccolite; (2) the lower diabase and the upper one now form- 
ing the cap may be two separate sills at different horizons. 
Minor intrusions and dykes are numerous all over the 
eastern and south-eastern parts of the Island, converting 
