DIABASE OF TASMANIA. 259 
laws of mountain elevation. They have no regular trend, 
and most of them are isolated without any systematic 
arrangement. Where there is the semblance of continuity 
in any one direction, a broken or deeply undulating outline 
suggests eruption from numerous vents along a line of fis- 
sure. As typical instances of the more prominent of these 
mountains, Ben Lomond may be cited in the north, and Mt. 
Wellington in the south, which, in common with the rest, 
are more or less bounded by a fringe of Permo-Carboniferous 
rocks, sometimes supporting conformable coal-measures. 
The actual contact line is everywhere hidden from view by 
dense vegetation, or accumulations of talus, the latter a 
natural consequence of the disintegration of cliffs of 
columnar diabase exposed by erosion of an original more or 
less rounded mass. In the Fingal valley the removal of 
the more or less flat-bedded sediments has in places bared 
the Silurian strata on which they rest. Sometimes they 
have been so faulted that they lie in successive benches on 
the flanks of both mountains; sometimes they dip away 
from them; while other masses of the same rocks at no 
* great distance appear wholly undisturbed. Elsewhere, and 
this applies almost to the whole mountain system of Eastern 
Tasmania, the forces operating in the uplift from below 
have opened fissures at the points or along the lines of least 
resistance, so that the molten magma has thrust itself in 
every direction, becoming visible, after cooling and subse 
quent denudation, in the form of sills and dykes at great 
distances from the main centres of disturbance. At one 
point on the northern flank of the Wellington Range, the 
removal of talus and forest growth by a great landslip in 
1872 exposed massive diabase sloping upwards at a mean 
angle of about 35° to a height of nearly 1000 feet, and the 
position of the altered and much-faulted sediments at and 
near its base suggests that this slope represents the original 
surface of the igneous mass in contact with the covering 
removed by local erosion. Space does not allow of any 
detailed account of the multitude of other somewhat isolated 
diabase-capped mountains, among which Mt. Field East, 
Mt. Field West, and Mt. Dromedary, Mts. Seymour, Tooms, 
and Connection, Mt. Arthur, and Quamby’s Bluff, to the 
south, east, north, and north-west respectively, may be just 
named as typical instances showing environment favourable 
to the theory of their laccolitic origin. And it may be 
noted that indirect evidence of a former covering of sedi- 
mentary rocks, or ac least of their contact with the diabase, 
is to be seen in the shingle and gravel beds distributed over 
