454 Bibliograp}ucal Notices. 



strata in sncccssional order from the Alongkian schistose formation 

 at the base, iip to the Benton (Cretaceous) beds at top. Thus 

 inchiding : ( 1 ) the Cambrian (after the Alougkian) ; (2) the Silurian 

 or Ordovician ; (3) the Carboniferous ; (4) the Permian and doubtful 

 Triassic; (5) the Jurassic ; and (6) the Cretaceous. There are some 

 Tertiary and Quaternary deposits, of limited extent, here and there 

 on the surface. 



The succession, chief characters, physical features, and relative 

 thicknesses of the several formations are given in the diagram at 

 page 18, and the table, pages 20 & 21 ; giving for these 

 Buperincumbent formations an aggregate thickness of more than 

 2000 feet. 



These are more or less completelj^ traversed by intrusive igneous 

 rocks, chiefly porphyritic, in dikes and sills, with laccolites. Tlie 

 intrusions were probably made in the Eocene period. But the 

 general uprise of the area was not caused by the forcible addition 

 of the porphyries and their associated intrusive rocks, but by other 

 earlier and different crush-movements, intimately connected with 

 the great movements that elevated the Bighorn Kange and 

 the Eocky Mountains. Many sections well show the extent of 

 erosion, exposing both the sedimentary and the igneous rocks, and 

 their local features ; also the particular crush-breccias of certain 

 old limestones. Careful calculations are made as to the formerly 

 superincumbent formations ; also as to the probable order of events, 

 and the local results of movements in the various schists and 

 limestones. 



The descriptive Geology of the mining districts (p. 33) treats 

 concisely of certain sections : — (I.) in the Alongkian Schists near 

 Homestake and Clover-leaf mines; (II.) in the Lower Palaeozoic 

 Eocks, (1st) on the railroad near Terry Peak, (2nd) near Englewood 

 in the Eo-Cambrian limestone, and (3rd) on the Bear-Butte Creek 

 near the town of Galena ; (III.) The Tertiary and Quaternaiy 

 deposits are not metalliferous, but some of the " ancient stream- 

 gravels may contain placer gold, but they have not been worked to 

 any extent, as compared with the gravels of the present gulches " 

 (p. 41). 



Part II. Mining Geology begins with the gold -ore of the 

 Alongkian rocks in the Homestake and Clover-leaf mines (pages 55- 

 94), treating in full of their history, development, geological features, 

 and the method of working; also of the character and constitution 

 of the ores and their associated rocks and minerals, and of the 

 probable order of deposition and metamorphiam. Traces of copper 

 are noticed in the schists at City Creek and Deadwood Hill. 



The ore-deposits in the Cambrian rocks of the Xorth Black Hills 

 (page 98, &c.) are considered both generally and in detail. Of late 

 years they have given an aggregate output nearly equal to that of the 

 Homestake Mine. They are the Gold and Silver ores of the Cam- 

 brian basal conglomerates (pages 104, &c.), the "Eefractory Siliceous 

 Eocks" (page 101, &c.), the Tungsten ores '^page 162, &c.), and the 

 Lead and Silver ores (page 169, &c.). The very interesting accouut 



