IGNEOUS ROCKS OF TASMANIA. 279 
None have been found yet in sub-basaltic drift; one was 
found 5 or 6 feet deep in the clay capping of tin-bearing 
drift, and the evidence points to them being younger than 
middle Tertiary. 
It has been surmised that they have been introduced into 
the Island and distributed by the aborigines, but this theory 
must be rejected in the face of their discovery at a depth of 
several feet from the surface, in one instance, at Lisle, in 
gold-bearing drift 18 feet below heavily timbered soil. 
The difficulties surrounding an explanation of their 
occurrence have given birth to several theories of their origin, 
all more or less unconvincing. Some of the most acute 
minds in geological science have been at work upon the 
problem, without solving it. The theory of their cosmic 
origin has lately been advanced, and re-inforced by some 
weighty considerations. Its advocates are Verbeek, Suess, 
P. G. Krause, Moulden, Walcott. I think before resorting 
to this explanation we must be quite sure that we have ex- 
hausted terrestrial sources. The suggested terrestrial sources 
are—l. Bubbles in lava; 2. Rocks vitrified by lightning ; 
3. Former lava sheets, now denuded; 4. Geysers; 5. Acid 
segregations in basaltic lava; 6. Volcanoes in New Zealand 
or the Antarctic. 
Mr. F. Stephens obtained an opinion from the late Pro- 
fessor Le Conte, of California, who advanced a suggestion 
by Professor A. C. Lawson, to the effect that the bombs 
might be the result of the bursting of bubbles on the surface 
of some liquid, stiffly-viscous lava, ready co solidify. The 
suggestion fails to account for the round form of the under- 
surface, and seems irreconcilable with the existence of steam 
pores, both superficial and internal. The theory also leaves 
the question of the origin of the lava quite untouched. 
Mr. Frank Rutley, an authority on fulgurites, throws out 
a suggestion that the objects may be fragments of rock 
struck by lightning and vitrified. The purity of the glass 
no doubt suggested the idea. The similarity of shape and 
sculpture negative this, and a glance at the bombs is suffi- 
cient to dispel it. 
The supposition that acid lava sheets formerly existed and 
have since utterly disappeared, derives no support from 
Tasmanian geology, and may be dismissed decisively. We 
have no signs anywhere of Tertiary acidic volcanoes. The 
improbability of these buttons being the sole survivors of 
such eruptions is very great. 
A suggestion has been made to me recently that the but 
tons may be drop-like forms of siliceous sinter or geyserite. 
