MINES 280 



limited is the field for the formation of fissures ; but it must be freely confessed that, 

 in the case before us the corrugation of half the continent into parallel ranges offers 

 good grounds for the expectation of vast longitudinal systems of fissures. When wo 

 come to consider the filling of these fissures, however, it is evident that the mineralo- 

 gical character of the vein-material must vary, to some extent, as to the gangue, but 

 to a still greater extent as to the nature of the ores. Even single mines, in the course 

 of extensive exploitation, have produced ores differing as widely as do those of tho 

 different zones enumerated by Mr. King. I am, in fact, strongly inclined to consider 

 freedom from base metals, for instance, a peculiarity due in many cases to secondary 

 processes, and not to be relied upon as characteristic for single veins even, to say 

 nothing of whole groups, districts, and continental zones. 



Nevertheless, the generalisations of Professor Blake and Mr. King on this subject 

 are highly interesting and valuable. The criticism here made is not in opposition to 

 their views so much as in qualification of a possible rash application on the part of the 

 general public. The zonal parallelism does exist, though in a somewhat irregular 

 way ; and it is clearly referrible, as these writers have shown, to the structural fea- 

 tures of the country, the leading feature of which is the longitudinal trend of the 

 mountain ranges. 



Subordinate to this trend (or, more strictly, resulting from the same causes as 

 produced it) appear the predominant longitudinal strike of the great outcrops of 

 sedimentary rocks, the longitudinal axes of granite outbursts, and, finally, the 

 longitudinal vents of lava-overflows and the arrangement of volcanos in similar 

 lines. It is evident that in crossing the country from east to west we traverse a 

 series of different formations ; while, by following routes parallel with the main 

 mountain ranges, we travel upon the continuous outcrops of the same general age. 



Mr. King distinguishes in the history of the entire Cordillera two periods of 

 disturbance, which have been accompanied by the rending of mountain chains, and 

 the ejection of igneous rocks. Such periods would afford the conditions of solfataric 

 action, thermal springs, and the generation of acid gases and metallic sublimates 

 and solutions, and thus favour the formation of metalliferous deposits. The first 

 of these periods, he says, culminated in the Jurassic, produced over the entire system 

 a profound disturbance, and is, in all probability, the dating-point of a large class of 

 lodes. To the second, or tertiary period, he assigns the mineral-veins which traverse 

 the early volcanic rocks. 



The expression ' culminated in the Jurassic ' merely refers, no doubt, to the fact 

 that the cretaceous strata of California repose unconformably upon the upturned and 

 metamorphosed Jurassic slates, having been themselves neither tilted nor highly 

 metamorphosed. Perhaps it is well to remember, however, that the cretaceous is a 

 weak point in the California series, at least, as determined by leading fossils ; and 

 perhaps the results of more complete stratigraphical surveys will indicate that there 

 are gaps of no little significance, dynamically and chronologically, in this part of the 

 geological record. At all events, the period of the folding of the Sierra Nevada (pre- 

 sumably that of the formation of many metalliferous deposits) was in some sense post- 

 Jurassic rather than Jurassic ; and probably this is the meaning of Mr. King, who 

 speaks of it in another passage as ' late Jurassic.' 



The lodes which are referred to this period are of two types : first, those wholly 

 inclosed in the granites, the outburst of which accompanied the upheaval of the earlier 

 stratified group, or in the metamorphosed Jurassic or sub-Jurassic strata ; secondly, 

 those which occupy planes of stratification or jointure, thus following in general the 

 dip and strike of the country-rock, while they present in other respects the indications 

 of fissure-veins. The veins of the Reese-river granite are examples of the first type ; 

 many gold-veins of California, the Humboldt mines, &c., are given as illustrations of 

 the second. The "White Pine district, the mineral deposits of which are said to be 

 inclosed conformably between strata of Devonian limestone, is declared to be a ' pro- 

 minent example of the groups 'comprised wholly within the ancient rocks.' 



We have hitherto supposed the strata immediately overlying the argentiferoiis 

 limestone at White Pine to be deep-water Carboniferous ; but their Devonian cha- 

 racter seems to be demonstrated by Mr. Arnold Hague. 1 More practically important 

 is tho assignment of these deposits to the earlier period of geological disturbance. 



Mr. King appears here to include in one group all the White Pine deposits, the 

 ' Base Range,' as well as ' Treasury Hill ' ; yet the striking distinction in mineralo- 

 gical character is worthy of regard. The deposits of Treasury Hill are notably free 

 from base metals ; and it seems to me that in their present form they must be due to 

 a secondary action, which has concentrated and recombined the metallic elements of 



1 See volume on ' Mining Industry of the United States Geological Exploration of tho Fortieth 

 Parallel.' 



VOL. III. U 



