THE GENESIS OF ORE DEPOSITS 489 
basis. This impossibility is inherent in the nature of the subject, 
since in nature there exist no sharply defined categories, no pigeon- 
holes into one of which every type will fit. It is the intermediate 
and transitional types that are the bugbear of any such attempt. 
Nevertheless there are some basic facts that may be used as a 
groundwork for generalization. In the first place we have certain 
ores occurring as original minerals in igneous rocks, and as dis- 
seminations and magmatic segregations whose origin from magmas 
is beyond doubt. Also there are the innumerable instances of 
ore-bearing pegmatites which can also be assigned with safety to 
the same origin. A great number of contact and replacement 
deposits also undoubtedly owe their metal content to transfer 
from intrusive masses. 
It is, however, when we come to the large and highly important 
class of deposits described in general terms as veins and lodes that 
difficulties begin to manifest themselves. These undoubtedly 
grade on the one hand into the magmatic deposits, while on the 
other hand some of them show distinct evidence of having been 
formed near the earth’s surface at the ordinary temperature and 
pressure. In dealing with the doubtful members of this group we 
have to take into account not only the characters of the deposits 
themselves, but all the attendant circumstances which may throw 
any light on their origin, such as geographical distribution, relation 
to sedimentary and other formations of known age, and to the 
structure, disposition, and character of the surrounding rocks; in 
short, their geological features. It is the geology of the ore cee 
that will throw most light on their origin. 
As an example let us take the mining region of western Corn- 
wall, one of the most highly mineralized districts of the world, 
especially as regards the number of metals found. Here cassiterite 
occurs as an original mineral in the granite, in pegmatites, in 
greisens and other pneumatolytic modifications of the granite, in 
quartz-porphyry dikes, and, most important of all, in a vast number 
of lodes and veins which chiefly congregate near the contact of 
granite and slate, almost invariably passing from one rock to the 
other without interruption. The lodes also show every possible 
degree of pneumatolytic alteration. Besides cassiterite they 
