408 R. H. RASTALL 
country rock. The endless varieties of contact deposits are not 
here taken into account in detail, but it may perhaps be said that 
they are more characteristic of acid intrusions, doubtless owing to 
the abundance of mineralizers, which act as carriers for the metals. 
Having thus considered in general terms the phenomena of 
differentiation and concentration in igneous rocks in their bearing 
on the origin of ore deposits, it remains to see whether it is possible 
to draw up a schematic classification on this basis. If this can be 
done it would constitute a genetic classification of ore deposits in 
the strictest sense of the word; such a classification, if presented in 
a graphic form, may be regarded as a genealogical tree of the ore 
deposits, or rather of the metals, as for this purpose it is hardly 
necessary to take into account their state of combination; further- 
more it is obvious that the characters and distribution of the great 
class of secondary deposits have no bearing on the point: it is 
with primary ores alone that we are concerned. Now most primary 
ores are either native metals, oxides, or sulphides—in some instances 
arsenic also seems to play the part of an electronegative element, as 
in enargite and several nickel and cobalt minerals and in arseno- 
pyrite. Furthermore, most primary ores are of very simple com- 
position: the more complex minerals are mostly found in the 
oxidized ores and in those of the zone of secondary enrichment, 
with which we are not here concerned. 
The primary facts that we have to start on are somewhat as 
follows : : 
t. It has been shown that certain metals and non-metallic 
elements are found chiefly in the basic rocks, including espe- 
cially nickel, cobalt, chromium, platinum, titanium, with chlorine, 
phosphorus, vanadium, and sulphur. Iron sulphides, often nickel- 
iferous, are common. 
2. Of these, chromium and platinum are specially characteristic 
of the ultrabasic rocks, themselves usually differentiates of basic 
magmas. 
3. In connection with the extreme differentiation phases of acid 
magmas are found especially tin, tungsten, molybdenum, zirconium, 
uranium, lithium, with fluorine, boron, and abundance of water. 
4. The rare earths of the thorium-cerium group are found 
chiefly in pegmatitic phases of granitic and syenitic magmas. 
