500 RK. H. RASTALL 
features it is similar to a diagram already published by the writer. 
The present form of it is, however, slightly more elaborate and the 
various branches have been arranged in accordance with the silica 
percentage of the rocks, although the figure must not be regarded 
as drawn strictly to scale. It is also to be remarked that the 
thickness and length of the branches do not bear any relation to 
the actual abundance of the different rock types: in this respect 
the figure is purely diagrammatic and not quantitative. The 
angles of inclination between the various branches are controlled 
only by convenience of drawing. ‘The intercrossing of various 
branches is, however, intentional, in order to show that similar 
final products may be obtained from different partial magmas. 
That is to say that similar rocks and ore types may have different 
chains of descent from a common ancestor. This is analogous 
to the biological phenomenon known as heterogeneous homoeo- 
morphy. 
This diagram, when looked at from this point of view, is in 
fact a basis for a truly genetic classification of certain large groups 
of primary ores; nevertheless it must be regarded as strictly limited 
in its scope. It takes no account of the secondary ores, or of 
certain groups which may be of supergene origin, formed by the 
action of descending meteoric waters. With these classes we are 
not now concerned. But it is claimed that this systematic arrange- 
ment does throw some light on the origin and genetic relationship 
of the great classes of primary ores, which are of fundamental 
importance, as being in all probability the original source of all 
the workable metallic deposits of the globe. 
In this treatment of the subject it is necessary to take into 
account also the conceptions of metallogenetic epochs and metallo- 
genetic provinces, which have been so ably worked out of late 
years by many writers, especially in America. If the whole scheme 
of differentiation as here outlined is regarded as continuous, a false 
impression will be obtained. On the contrary, the processes are 
discontinuous both in time and space. An admirable summary 
of the present state of our knowledge of the chronology of ore 
™ Rastall, ‘Differentiation and Ore-Deposits,’’ Geological Magazine, Vol. LVI 
(1920), p. 298. 
