ABSTRACT OF PROCEEDINGS. 
XIII. 
Mr. L. C. Ball briefly described the geological conditions at 
Cairncross Dock on the Brisbane River, and exhibited fossil plants 
collected by him during excavation. 
Mr. 0. A. Jones spoke of the fossils collected by Mr. Ball. A number 
of the common species of the Ipswich Series were found, but most interest- 
ing was a new species probably of the genus Schizoneura. This was 
larger than any described from Mesozoic deposits anywhere in the 
world. Many fragments of this were found, including one with the 
undivided leaf sheath attached. It is probable that most of the structure 
of the form can be reconstructed from the specimens. The assemblage 
indicates that the beds are part of the Ipswich Series. 
Mr. J. T. Woods exhibited several specimens of two new species of 
Equisetites from Brighton, near Sandgate. The material, though frag- 
mentary, is very well preserved, and this has enabled much fine structure 
to be worked out, even the outlines of the cells of the epidermis of the 
sporangia being observed in one instance. Fertile specimens of 
Equisetites are uncommon, and the detail with which the structure of the 
cones and sporophylls can be worked out makes the find an important 
one. 
Dr. H. I. Jensen gave an address on the Origin and Distribution of 
Mica in Northern Australia. He first outlined the nature and composi- 
tion of the various classes of mica. Thereafter he gave the properties 
and more common uses of the commercial micas. The mode of occurrence 
of phlogopite in Central Australia was fully described, and its origin 
was attributed to alteration of olivine is an ultrabasic dyke by potassU 
solutions from later pegmatite and aplite intrusions. The occurrence of 
large books of muscovite in certain very coarse pegmatitic dykes of acid 
nature, was fully described. As regards the origin of large books of 
muscovite it was shown that pneumatolytic pegmatites crystallise in the 
same way as water solutions under high temperature and pressure, and 
the sequence of crystallisation of the minerals in them is entirely different 
from that which obtains in muscovite granite. The sequence of consolida- 
tion in the mica-bearing pegmatites is (1) the quartz of the massive 
quartz blows, (2) a eutectic of graphic quartz and felspar, (3) massive 
felspar, (4) an aggregate of crystals of quartz, felspar and mica, and 
(5) lastly muscovite books and glassy quartz. The reasons for this 
aberrant mode of crystallisation are the presence, in the mother-liquor 
which forms the pegmatite, of abundant water and carbon dioxide gas. 
The escape of much of the gas into fissures causes the early precipitation 
of much of the quartz in solution. After that the sequence of crystallisa- 
tion is governed by the following factors: — ( a ) Specific gravity, 
(6) formula-volume, (c) oxide volume, ( d ) molecular volume, 
(e) pressure, (/) temperature, and (g) stress. Potash, being the alkali 
oxide with the highest oxide volume, enters into combination later than 
the other alkali oxides, in fact, last. It is still in solution when the zones 
of the earth’s crust intruded has cooled sufficiently to come into the zone 
of stress, for which reason, together with the presence of water, mica, a 
hydrous stress mineral forms in preference to orthoclase. The potassic 
solutions corrode and absorb earlier-formed felspar to form mica and 
quartz, if sufficient alumina is not present in the residual magma. 
Various mica mines in the Territory were described and the future of 
mica mining was also touched on. 
