382 SECTIONAL TRANSACTIONS.—C. 
Permian times, of magmatic conditions under the Pennines, which may have been 
directly or indirectly responsible for the localisation of the ore deposits. The presence 
of fluorspar in great quantity, suggests, by analogy with Cornwall, derivation from 
an acid magma; and indeed, the whole concentrated assemblage of rare elements 
supports this conclusion. 
The chief line of evidence against hypogene origin lies in the impoverishment of 
the veins in depth, which is without doubt a real fact. It is clear, however, that the 
veins do not disappear, and it is suggested that local concentration is due to interaction 
between hypogene solutions and ground waters. 
Prof. P. Nie@e@xt. 
I. The scope of the term ‘ Useful mineral deposits’ is a variable function depending 
on world economics and world industry. ‘Thus, it did not come to include nickel-bearing 
deposits until the nineteenth century. The necessary minimum tenor for exploitation 
varies in very wide limits from element to element and may, even among the ordinary 
metals, be a million times greater in one case than another. A comprehensive term 
such as ‘ mineral deposits’ should, therefore, never be used in the exclusive sense of 
workable mineral aggregates, for the separation of these from the rest must necessarily 
seem arbitrary from the scientific point of view. 
Il. In all new classifications of ore deposits attention is paid to questions of genesis 
as being the most important for the prospector and practical investigator. So long 
as the anthropological standpoint which distinguishes between economic and non- 
economic deposits is retained, it seems as futile to expect a solution of the genetic 
problems attaching to mineral associations as it would be to hope to deduce a general 
theory of evolution in zoology from the study of, say, noxious insects alone. 
Ul. The chief processes of mineral formation produce mineral deposits of rock- 
character. The special deposits mined for certain elements and minerals are generally 
by-products of rock-forming or rock-altering processes. The technically important 
parageneses of elements or minerals appearing en these deposits must be accounted 
for by general geochemical laws. Rock-forming processes and those leading to the 
formation of local accumulations or concentrations of useful elements can only be 
understood in their full significance if the natural mineral associations are regarded 
as an entirety, if rock deposits and mineral deposits be accepted as parts of a whole. 
From the practical point of view this is important, as observations on rocks or valueless 
mineral occurrences may indicate the whereabouts of workable deposits. 
IV. Connections can always be established between economic deposits and one 
or more of the following major processes of rock-formation. 
A. Processes of magmatic differentiation and consolidation. 
With these are connected concentrations of matter : 
(a) during the chief period of consolidation of igneous rocks : 
Orthomagmatic. 
(6) under the influence of the gradual concentration of volatile constituents 
at high temperature : 
Pegmatitic-Pneumatolytic. 
(c) in aqueous residual solutions, exhalations, thermal waters, etc. : 
Hydrothermal. 
B. Processes of sedimentation (resulting from the interaction of the rock mantle 
with the atmosphere and hydrosphere). 
With these are connected concentrations of matter : 
(a) in solutions arising from weathering processes and in the products of 
precipitation of these ; 
(6) in the residues arising from weathering processes and in the products of 
their transportation. 
C. Processes of metamorphism (resulting from the influence of external factors 
excluding the atmosphere and hydrosphere, on pre- 
existing mineral associations). 
In connection with these, matter may undergo : 
(a) relative concentration through loss and differentiated migration of certain 
substances ; 
(6) concentration by lateralsecretion during geological processes which affect 
the country rocks. Crystallisations from descending solutions in the 
epi-zone. 
Be a | a 
