MINING DISTEICTS. 7 



Many important veins of Mexico, several of those which border upon the 

 Colorado Eiver within the United 'States, and, in general, that zone which lies 

 along the eastern base of the Sierra Nevada, are members of this family, as 

 is clearly proven by the fact that they are either wholly, or in part, cased by 

 volcanic rocks. The most prominent example of this type, within the limits 

 of this exploration, is the Washoe disti'ict, whose remarkable Comstock lode, 

 although in one place indistinctly touching the ancient formations, yet, as will 

 hereafter be seen, is chiefly inclosed by a modem volcanic rock, and evidently 

 owes its origin to the later disturbances. The Owyhee district, in Central 

 Idaho, occurs upon the crest of a granite mountain chain, which has been 

 intersected by a series of volcanic dikes, ranging from the earliest propylite to 

 the most recent basalt. From the peculiar association of the mineral veins 

 with these dikes, and the manner in which they intersect each other, it is 

 obvious that the quartz lodes belong to the Tertiary period. 



From these few but important general facts, it will be seen that by far 

 the greater number of metalliferous lodes occur either in the stratified meta- 

 morphic rocks or in those ancient eruptive rocks, which date from the period 

 of Jurassic upheaval; yet very important, and, perhaps, more wonderfully pro- 

 ductive, have been those silver lodes which lie wholly in the recent volcanic 

 formations. It is evident from a careful study of the ranges that much of the 

 dislocation and general mountain disturbance was occasioned by the Tertiary 

 upheavals. How far the veins which lie wholly in the older rocks belong to 

 this second disturbance it is impossible to say. In some instances it is evi- 

 dent that the veins themselves have been twisted and disturbed together with 

 the strata; in others it seems most probable that the whole fissure and solfatara 

 were induced by the latest movement. In the present state of knowledge, it 

 is impossible accurately to classify and catalogue the age of the fissures them- 

 selves. In almost all cases in which mineral districts are described in this 

 volume, the age of the inclosing rocks is clearly laid before the reader, but 

 data are frequently wanting as to the age of the formation of the fissure. In 

 a majority of cases the evidence tends to the belief that the veins belong to the 

 Jurassic period ; and yet it should be borne in mind that wherever the more 

 recent strata have been formed from the detrital materials of the older, we look 

 in vain for the ore-bearing pebbles. The writer is not aware that even in the 



