466 



NATURE 



[March 19, 1896 



are at first rather misleading, as one scale is employed 

 for the measures ; another and much larger one for the 

 interbedded coal-seams. It is pointed out that in many 

 cases it is not the only, nor indeed always the best, seam 

 which is actually mined at any particular spot. 



Passing to the west side of the continent, we have 

 first to deal with the work of Mr. H. W. Turner on the 

 rocks of the Sierra Nevada.^ The older set of rocks, 

 which contain auriferous veins, and are spoken of as the 

 auriferous series, are much disturbed, and it is very 

 difficult to make out their exact relations, particularly as 

 they are bent into a series of isoclinal folds. Fortunately, 

 however, many of the beds contain fossils, by the aid of 

 which it may be possible eventually to unravel their 

 structure. The following. Systems appear to be repre- 

 sented : the Silurian, Carboniferous (three divisions), 

 Jura-Trias (?), Trias (two divisions), and Upper Jurassic 

 or Lower Cretaceous. The last, or Mariposa formation, 

 appears to overlap the others, and sometimes to rest on 

 Carboniferous beds ; but the maps which express this 

 fact are not easy to interpret. These older rocks contain 

 beds of lava and ash at many horizons, with intrusive 

 rocks of all types, from granulite and granodiorite (or 

 quartz-mica-diorite) to peridotites and pyroxenites. The 

 sediments are often highly metamorphosed, and, so far 

 as we may judge by the nature of the minerals produced, 

 such as andalusite and sillimanite, in all probability by 

 thermal action. 



The newer rocks include representatives of the Upper 

 Cretaceous, Eocene, Miocene, Neocene, Pliocene, and 

 Pleistocene Systems. The Neocene rocks are the auri- 

 ferous river gravels, and the Pliocene beds are shore 

 gravels, which sometimes contain gold. These are 

 associated with flows of rhyolite, basalt, and andesite 

 of various types, and with great sheets of breccia and 

 tuff derived from them. The Sierra appears to have 

 received its chief uplift after the deposit of the Mariposa 

 slates, and then to have been planed down in both 

 Cretaceous and Tertiary times to an almost flat sur- 

 face, which is in places preserved under the Tertiary 

 volcanic accumulations. Late in Tertiary time came 

 another great uplift, accompanied by faulting. 



A very useful piece of economic work is that by 

 Mr. W. Lindgren on the gold-silver veins of Ophir, 

 California.^ The country-rock consists of an area of 

 schistose amphibolites towards the north-east, into which 

 a mass of granodiorite was intruded at some date subse- 

 quent to the formation of the Mariposa slates. The 

 amphibolites contain lath-shaped felspars set in horn- 

 blende, chlorite, and felspar, and they have evidently been 

 derived from the alteration of diabases ; the grandiorite 

 consists of quartz, potash and soda-lime felspar, biotite, 

 and hornblende. Dykes of amphibolite and the auri- 

 ferous veins traverse both rocks, and the latter group them- 

 selves into four systems : a north-west and south-east 

 set parallel to the strike of the schists, another set running 

 north and south, a third west io° north, and a fourth east 

 20° north. To some extent all the quartz veins bear 

 precious metal, but the ore is concentrated into pockets 

 and chimneys. While the veins in the granodiorite are 

 more extensive, of low grade and rich in silver, those in 

 the amphibolite are of smaller extent in depth and width, 

 but richer in gold, poorer in silver, and frequently 

 reduced to mere pockets. The amphibolite is in places 

 impregnated with pyrites, and it is well known that where 

 the veins cross these " iron belts " they become especially 

 rich in ore. The minerals include native gold, silver, and 

 copper, electrum, horn-silver, and many sulphides. 

 Both types of rock are much altered along the walls of 

 the veins, the amphibolite passing into an aggregate of 

 carbonates, white mica, and pyrites. 



The geology of the Eureka district, Nevada, by Mr. A. 



1 Fourteenth Annual Report of the Geological Survey of the United 

 States, 1892-93. (1894.) 2 iliid. 



NO. 1377, VOL. 53] 



Hague,^ contains an appendix on the fossils by Mr. 

 Walcott, and another on the igneous rocks by Prof. 

 Iddings. The work begins with a general description of 

 the physical geography ajid geology, from which we 

 gather that the following is the succession of the chief 

 rocks of the district. The Cambrian rocks are 7700 

 feet thick, and are divided into the Prospect Mountain 

 quartzite, the Prospect Mountain limestone, which con- 

 tains an Olenellus fauna, the Secret Caiion shale, the 

 Hamburg limestone with a Potsdam fauna in its lower 

 and upper parts, and the Hamburg shale. The Silurian 

 rocks are 5000 feet in thickness, made up of the Pogonip 

 limestone with a Potsdam fauna at the base, then Chazy 

 forms, and in the higher portion some of Trenton facies ; 

 the Eureka quartzite ; and the Lone Mountain limestone 

 with Trenton and, possibly, some Niagara fossils. The 

 Devonian system is 8000 feet thick, divided into the 

 Nevada limestone and the White Pine shale. The 

 Carboniferous Formation has four members, the Diamond 

 Peak quartzite, the Lower Coal Measure limestone, the 

 Weber conglomerate, and the Upper Coal Measure lime- 

 stone. The chief feature about the Carboniferous rocks is 

 the fact that there occurs a freshwater fauna at the base 

 of the Lower Coal Measure limestone, and a mingling of 

 Devonian, Lower Carboniferous, and Coal Measure 

 species in a limestone overlying beds characterised by a 

 pure Coal Measure fauna. 



These sediments are penetrated by granites, granite- 

 porphyries, and quartz-porphyry, and are overlain by 

 immense quantities of Tertiary or later volcanic rocks. 

 The great eruptions for the most part cover subsided 

 regions ; where a mountain block has been uplifted, they 

 occur at its borders. The succession of eruptions seems to 

 have been as follows : hornblende-andesite, hornblende- 

 mica-andesite, dacite, rhyolite, pyro.xene-andesite, basalt. 

 It is suggested that the original magma split up into 

 two — one felspathic, the other pyroxenic ; the felspathir 

 eruptions become steadily more acid until rhyolite is 

 reached, the pyroxenic eruptions more basic from 

 andesite to basalt. The ore deposits are mainly of lead 

 and iron rich in gold and silver. Originally deposited as 

 sulphides, they have been oxidised hy air and surface 

 water. They occur in cavities in all the Cambrian, 

 Silurian, and Devonian rocks except the great shale 

 masses. One analysis gave 27*5 ounces of silver and i'5 

 of gold to the ton. 



Mr. Walcott's appendix includes a long list of fossils 

 from all the beds. Mr. Iddings calls the granite an 

 amphibole-granitite containing quartz, orthoclase, plagio- 

 clase, hornblende, and mica ; the granite-porphyry is the 

 chilled-edge phase of the granite-magma, of which the 

 quartz-porphyry is the final phase. In all the volcanic 

 rocks " there is a marked similarity between the individual 

 crystals of the same mineral species wherever they occur 

 . . , which links the various kinds of rocks together 

 and suggests the possibility of a common source." The 

 work is admirably illustrated by figures and photographs. 



Mr. J. S. Diller gives an impressive picture of Tertiary 

 denudation in his "Tertiary Revolution in the Topo- 

 graphy of the Pacific Coast." ^ After the beds of the 

 Shasta-Chico (Cretaceous) series were laid down, the 

 country was planed down to a base-level. At the present 

 time this " pene-plain " has an average maximum slope 

 of 2°, which rises to 5°, and then somewhat rapidly up 

 to 17°, on the flanks of the Klamath Mountains, a name 

 used to designate the group which occurs at the junction 

 of the Cascade range with the Coast range and the 

 Sierra Nevada. The plain stretches across the head 

 waters of the Sacramento River to the Sierra Nevada, 

 and probably comes out at the other side of it in the 



1 Monographs of the Geological Survey of the United States, vol. xx. 



2 Fourteenth Annual Report of the Geological Survey of the Unite! 

 States, 1892-93. ■ (1894.) 



