380 SECTIONAL TRANSACTIONS.—C. 
(Forbes) and Cyprione bristovit (Jones), these are followed by an early form of Cypridea 
dunkeri (Jones) and Cypridea punctata (Forbes), which are later in the phase replaced 
by Cypridea granulosa (Sow.), these latter becoming more abundant as the water 
became more brackish. 
The early form of C. granulosa had apparently been evolving elsewhere during the 
long fresh-water period which initiated the third phase. The form which appeared 
in the middle of this phase being a new variety, C. granulosa var. paucigranulata 
(Jones), which, as the water became more saline, gradually changed into the form 
C. granulosa var. fasciculata (Forbes). 
The third phase was brought to an end by a marine invasion and the deposition 
of great banks of Ostrea distorta (the ‘ Cinder Bed ’). 
Tuesday, September 29. 
Discussion on The Genesis of Ores in relation to Petrographic Processes. 
(Prof. C. G. Cuttis; Dr. A. BRammatzi; Mr. K. C. Dunnam ; Prof. 
P. Nieeii, Dr. W. R. Jones.) 
Prof. C. GILBERT CULLIS. 
The origin of ores has long been a subject of speculation. The terms ‘ neptunist,’ 
‘ vuleanist,’ ‘descensionist,’ ‘ascensionist’ and ‘lateral secretionist —connoting 
deposition from an ocean, by igneous injection, or from solutions travelling downwards, 
upwards or laterally—-recall early genetic hypotheses. These theories, though partly 
valid, were parochial in character because based upon restricted observation, With 
fuller knowledge of rocks and rock formation broader views as to ores and ore formation 
have been evolved. 
Ore-deposits are local concentrations in which valuable metals have been segregated 
so as to be present in far larger proportions than in average rocks. The genesis of 
such bodies implies the geological process or processes by which concentration has 
been effected. 
These concentrations, being naturally-formed mineral aggregates, cannot logically 
be distinguished from rocks. They are highly specialised rocks, however, as is 
indicated by their mineral composition and rareness, and by their small dimensions 
as compared with ordinary rock masses. Regarded in this light they may be 
genetically referred to the three great modes of rock formation and classified as igneous, 
sedimentary and metamorphic. 
Deposits of igneous origin exceed all others in importance ; indeed, the study of 
the genesis of ores is in the main the study of the processes of concentration in, or in 
association with, igneous rocks. : 
Whether the metals were inherent to the igneous material or whether introduced 
from a deep-seated ore-horizon, their segregation into ore-bodies seems to have been 
due to the extreme action of magmatic differentiation. 
During this process—by which an original magma separates into sub-magmas— 
certain metals, ¢.g. platinum, chromium, titanium, iron, nickel, copper, &c., migrate 
with the basic fraction, and under favourable conditions are concentrated by gravity 
in magmatic ore-bodies at or near the floors of gabbroid intrusions. Others, e.g. tin, 
tungsten, molybdenum, bismuth, iron, copper, gold, zine, lead, silver, antimony, 
mercury, &c., migrating to the acid fraction, form fugitive compounds with water 
and other volatiles and undergo anti-gravity concentration at or near the roofs of 
granitic cupolas, as pegmatitic, pneumatolytic, or contact deposits, or pass in hydro- 
thermal solutions up into the surrounding country-rock and are deposited, in a well 
defined temperature sequence, as cayity-fillings or replacements at various distances 
from the parent igneous rock. 
It is these hydrothermal solutions that have formed most mineral veins, and since 
filling has been effected by magmatic emanations such veins may reasonably be 
regarded as igneous intrusions. 
Ore-deposits of sedimentary origin are merely special cases, either of detrital rocks 
resulting from the disintegration of pre-existing ore-bearing formations, or of chemical 
sediments precipitated from surface waters. In the former type there has been 
gravity concentration of certain economic minerals, e.g. platinum, gold, tinstone, &c., — 
