IGNEOUS ROCKS OF TASMANIA. 289 
v4 
sandstones into quartzite and adinole, the latter a cherty or 
flinty looking rock, which furnished the aboriginals with 
much of the material which they chipped into the forms of 
their rude flakes. The coal seams in the neighbourhood 
of these dykes are injured by the proximity of the intrusive 
rock, the coal being rendered brittle, stony, and unfit for 
use. 
Our views of the probable extension of our coal-measures 
will be materially affected by the theory we form of the 
occurrence of the diabase. If the apparent igneous caps be 
real caps, our coal seams will pass through the mountain 
ranges underneath the horizontal sheets of diabase; but if 
they are only apparent, the area of our coal-bearing strata 
will be wonderfully curtailed. From a practical as well 
as from a scientific pomt of view, it would be interesting 
to see some deep bores put down into the igneous rock which 
crowns the Tiers. If these reached sedimentary rock 
below, boring for coal where the cap is likely to be thin 
would be undertaken freely. 
Opuitic STRUCTURE. 
The diabase often has this strongly developed. In it the 
pyroxene forms a cement, enwrapping and moulding itself 
on che felspar prisms, which then have the appearance of 
cutting up a formless mass of augite into different sections. 
The augite, however, suffers no solution of optical continuity, 
as is easily seen by rotating ic between crossed nicols. 
Rosenbusch calls this the diabasic-granular structure. 
Fouqué and Levy had already called attention to it as 
ophitic, from che ophite of the Pyrenees. . In some of our 
diabase it is developed quite typically, and it is to be found 
im the more crystalline of our basalts, though among the 
latter the intersertal structure is more common. This is the 
structure in which an interstitial ground-mass makes its 
appearance. Rosenbusch remarks that the intersertal 
struccure is more usual in fresh diabases than in those found 
among the older rocks. If this be so, it is difficult to 
explain, for these structures must be considered as original, 
not superinduced after consolidation. Fouqué and Levy 
have reproduced it artificially in their synthetical experi- 
ments. Loewinson-Lessing (Bull de la Soc. Belge de Géol., 
&c., tome u., 1888, pp. 84-87) contends that it has been 
produced by lava-flow under the pressure of the sea. Now 
the ophitic gabbros of the Western Isles of Scotland (see 
Judd’s papers) have been shown to have had a sub-aerial 
. U 
