310 RESUME AND THEORETICAL DISCUSSION. 
influence in directing the erosive agencies by which, since the gravel epoch, 
the present relief of the surface has been developed. But it is necessary also 
to inquire whether there is not some more intimate connection of volcanic 
activity with the auriferous character of the gravels than is implied in the 
very obvious one of a protection against erosion. It would not be safe to 
assume that where volcanic rocks do not now exist, there may have been 
in former times bodies of gravel which would have remained so as now to 
be worked for gold, had the volcanic vents on the summit of the Sierra 
above them emitted the material necessary to form the protecting cover. 
The intimate connection of the occurrence of metalliferous deposits in 
general with the metamorphism of the associated rocks is something which is 
readily perceived, even if not thoroughly understood. That there is a more 
or less intimate relation between mineral veins and igneous rocks has also 
been recognized by some of those who have occupied themselves with the 
study of vein phenomena. For instance, it is a well-known fact that metal- 
liferous deposits are often developed in the immediate vicinity of, if not 
directly in contact with, dikes of volcanic rock; also that portions of a sedi- 
mentary formation, which are found to be metalliferous over areas where 
igneous agencies have been in operation, may become entirely barren at a 
little distance from these regions of disturbance.* 
It is not to be denied, however, that volcanic rocks are not usually metal- 
liferous; it is only those which have been metamorphosed which enclose 
veins of importance ; so that in this respect they hold the same relations to 
mineral veins that other formations do. The older any mass of rock is, the 
more likely it is to have undergone those chemical transformations to which 
is applied. But the Tertiary volcanic rocks, which 
> 
the term “ metamorphic’ 
make up so large a part of the Cordilleras, or at least of the superficial por- 
tion of some of its principal ranges, are too recent to have been subjected to 
much chemical change since their eruption; it is only here and there, as 
near Virginia City, that they have been greatly metamorphosed, and are, at 
the same time, highly metalliferous. 
That the débris of the voleanic rocks themselves in the Sierra are only 
very rarely found to contain gold, is a well-known fact. There are a few 
localities where it is said that washing gravel of this character has been 
* As an instance of this, the relations of the Potsdam sandstone in the vicinity of the line of volcanic activity 
extending through Keweenaw Point and on westward, in a line parallel with the shore of Lake Superior, may be 
mentioned. The development of the lodes at Przibram is an excellent illustration of the occasional remarkable 
association and interdependence of mineral veins and eruptive dikes. 
