IGNEOUS ROCKS OF TASMANIA. 287 
DraBaSE: Its GEOLOGICAL OCCURRENCE. 
Both Zirkel and Rosenbusch, as well as A. Lacroix, place 
diabase among effusive rocks, while stating that it occurs 
also as an intrusive. Its effusive habit need not detain us 
now, for all the signs of its occurrenee in Tasmania indicate 
its intrusion into overlying rocks. It contains no g:ass, no 
scoria, has no horizontal partings between separate lava 
flows, has no development of microlites or zeolites, is coarsely 
holocrystalline, sometimes almost gabbroid. It is found in 
Permo-Carbcniferous and Mesozoic strata, as well as in 
Devonian , granite, preserving a wonderful uniformity in 
structure and constitution, which would be truly singular 
if its occurrences in these different systems were prolonged 
contemporaneous lava-flows. Zirkel mentions that the 
effusive diabases carry on their surfaces fluidal signs, ropy 
twists, glass crusts, and are associated with tuffs. In spite 
of this, he goes on to say that, in respect of internal struc- 
ture, no thorough-going difference can be drawn between 
the effusive diabases and those which occur as intrusive 
sheets (Lehrbuch der Petrographie, vol. ii., p. 651). A. 
Lacroix, in his “Gabbro du Pallet et ses Modifications,” 
1899, pp. 27-30, has an interesting chapter on the terms 
gabbro and diabase. He adopts the English use of the 
term gabbro, which includes gabbro proper and a portion of 
the old French diabases (those with gabbroid structure). 
It is curious to note how usage in different countries affects 
nomenclature in different ways. M. Lacroix rejects the 
term ‘dolerite’? for nearly the same reason that has in- 
fluenced English petrographers in rejecting “ diabase,”’ viz., 
because the question of age is implicated. In France 
dolerite has been applied to the Tertiary rock and diabase 
to the preTertiary—hence modern French petrography 
drops the term dolerite. In England, as a protest against 
the same usage, diabase has been dropped by a <ection of 
English petrographers, and dolerite retained. Rutley 
uses the term dolerite to denote holocrystalline basic dyke 
and intrusive rocks which pass upward into the basalts. 
At certain points in their passage they may be regarded 
ag truly volcanic rocks. Harker calls the larger intrusive 
bodies of hypabyssal pyroxenic rocks diabase, and the minor 
intrusions of the same rock dolerite. 
The intermediate posicion of these rocks between plutonics 
and volcanics inevitably gives rise to varieties where the 
line is difficult to draw—such varieties are gabbroid and 
effusive diabases. The typical ophites, or diabases, of the 
Pyrenees are intrusive bosses, buc Lacroix also points out 
