258 PROCEEDINGS OF SECTION C. 
Henry Mountain bodies in mode of origin. The structure 
of the rock implies a deep-seated site of consolidation.” 
Speaking generally of the intrusive masses in the region 
under discugsion, Whitman Cross notes as a prominent fact 
that ‘‘ the great majority of them belong to one well-marked 
structural type, with but slight variation in mineralogical 
constitution.” The principal types of the rocks analysed 
are recorded as Augite-porphyrite, Hornblende-porphyrite, 
Quartz-porphyrite, and Hornblende-mica-porphyrite. The 
variable elements in the series are silica, iron, lime, and mag- 
nesia, while alumina and the alkalis remain nearly constant. 
In the augite-porphyrite there is no distinguishable quartz. 
As to the geological age of the laccolitic mountains, it 
appears to have been séttled that all the Mesozoic strata 
now remaining have been disturbed by the intrusion, which 
in some instances has affected Upper Cretaceous beds, its 
date being thus brought down to the Tertiary period. 
These records are the result of systematic geological 
examination of an immense area previously mapped out by 
an accurate topographical survey, with contour intervals of 
200 feet. What light can they throw on the history of 
igneous dykes and cappings in Tasmania, where we have 
no authentic data beyond the altitudes and approximate out- 
lines of some of the principal mountains, and the results of 
individual exploration in a few isolated districts! Such 
conditions afford no grounds on which to base any final con- 
clusions, but an actempt will be made in the remaining por- 
tion of this paper to show that the laccolite theory may be 
accepted as a reasonable explanation of the Tasmanian pro- 
blem. If the evidence is only of a circumstantial character, 
all of it, at any rate, seems to point the same way. 
As a chief cause of the stupendous development of vol- 
canic energy, as seen in the cappings, sheets, and dykes of 
diabase which have determined the physical contour of 
nearly the whole of the eastern half of Tasmania, Professor 
David long ago suggested heavy sedimentation during the 
ages following the folding of the Silurian and older strata. 
There are no data for determining the chickness of the 
Permo-Carboniferous series, but it is probably not less than 
several thousands of feet; that of the subsequent pre 
Tertiary sedimentation, including the remnants of it which 
are classed as Mesozoic coal-measures, is quite an unknown 
quantity, but may have been very considerable. 
The first point of resemblance to be noted between these 
diabase-capped mountains and the laccolites of the Rocky 
Mountain region is that they follow none of the ordinary 
