SECTIONAL TRANSACTIONS.—C. 385 
deformable metals. Pyrite, on the other hand, fractures under similar conditions, 
that is to say, it suffers clastic deformation. With minerals representative of the 
whole range of mechanical properties available for observation, it becomes increasingly 
evident how highly selective the action of dislocation metamorphism is. It will be 
necessary to investigate the variation among ore minerals of the susceptibility to 
plastic deformation and to work out the sequence of their tendency to crystalloblastic 
regeneration, Under the influence of differential movements ore solutions and plastic 
ore masses may migrate along fractures and fissures, but their tendency to do so is 
different, with the result that selective impregnations may result. Fragments of 
the country rock may be folded into the plastic ore masses if the stress be sufficiently 
strong, veins may be drawn out to lenses, etc. The resulting ore bodies are often 
less to be described as veins or beds than as mineralized zones with irregular distribu- 
tion of the ore. Thin and polished sections of the ore often present highly interesting 
aspects of the replacement, unmixing, folding and other processes which have taken 
place. In the deformed deposits of the alps, albite is frequently a characteristic 
newly-formed mineral, zincblende is usually poor in iron and light yellow in colour 
and, although the metamorphism took place at relatively low temperatures, magnetite 
and pyrrhotite are often present. A number of other typical features could be quoted 
for these deformed ore deposits of which the type-characteristics will have to be 
worked out by comparative study. It may already be said that nothing but a thorough 
knowledge of the mechanical, physical and chemical fundamentals of rock metamorphism 
will lead to a satisfactory understanding of these deposits and the great practical 
difficulties they often raise. 
VI. The natural outcome of a treatment of mineral deposits which draws no 
sharp line between rock formation and the formatiou of local (accessory) mineral 
deposits of economic or non-economic character is the conception of rock and mineral 
provinces. In this connection it is necessary to work out the relations between the 
various mineralizing processes in any given geological unit and to compare the results 
obtained for different processesin such natural units. Aninsight is thus obtained into the 
causality existing between geological and geographical factors and world economics. 
Tt may be recalled that in the circum-pacific area the mesoid major folding was 
accompanied by intensive magmatic movement, and consequently very extended 
ore mineralization. (North America, Central and South America, Japan, are all 
important ore areas.) In the alpine area, which witnessed the superimposition of 
two continents, magmatic solutions generally found their upward way barred. (The 
central alps are poor in young ore deposits.) Similarly, the extent and importance of 
sedimentary ore deposits and such connected with weathering, can only be fully 
estimated by attempts at reconstruction of the distribution of land and water and of 
the climatic factors, ete., which obtained at the time of formation. 
The reconstruction of the geological history of a given part of the earth’s crust 
with reference to the processes of mineral formation (including rock formation) which 
accompany every major geological act, enables a prediction to be made of the economic 
deposits which are possible and may be found in that part of the crust. Earth 
history, which devotes equal care to all types of mineral associations, is not merely a 
science which treats of the past. It is no less thestarting-point for the estimation of 
the world’s reserves of raw materials and for the framing of future economic policy. 
Dr. A. BRAMMAL. 
Magmatic ore-deposits are as closely the concern of the petrologist as are peridotites | 
and greisens, anorthosites and aplites: Prof. P. Niggli has shown that their problems 
are inseparable from those concerned with magmatic differentiation. 
But discussion of these problems hitherto has been based largely on a threefold 
assumption: that (a) differentiation is the main, if not the sole, process concerned in 
the evolution of igneous rocks ; (b) the primary magma is gabbroid or basaltic ; and 
(c) all other magmas are differentiates from this basic magma, the normal rock- 
sequence being peridotites, perknites, &c.—>gabbros —~ diorites ~ granodiorites 
granites or their extrusive phases. The conception is thus essentially restricted to 
that of a basic magma worked upon by an evolutionary process, i.e. to the initial 
raw material and the machine. 
Admittedly the machine is adjustable to tectonic and other factors at any stage 
of the process, and the raw material is not precisely standardised. But one is entitled 
to doubt whether this process alone, unaided by either assimilation or palingenesis, 
1931 ante. 
