





F»T 

















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 

 the term u metamorphic " is applied. But the Tertiary volcanic rocks, which 

 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 debris of the volcanic 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. 







