March 9, 1905] 



NA TURE 



451 



Geological Survey, all these stimulants are conspicuous. 

 The memoir on the Bitterroot Range alone deals with an 

 area of about 12,000 square miles, respecting which our 

 scientific knowledge has been hitherto of the scantiest ; 

 while the other three, though professedly more limited in 

 scope, treat in detail of areas ranging from about 450 

 to 560 square miles which may be taken as selected 

 illustrations of parts of the vast region west of the 

 Mississippi. 



Of course, it is not area only that counts in geology ; 

 and in considering the magnificent distances of the Great 

 West, we may take heart in that our own shreds of land 

 have not been carved out of some wide monotonous tract 

 covered by a single formation within which it might be 

 the fate of an ardent geologist of limited means to find 

 himself hopelessly tethered ! It is, indeed, fortunate that 

 in the ge_oIogical map of the world the British Isles 

 lie, as it were, athwart the index. 



It is less easy to find consolation when we compare 

 even the most presentable of our British geological publi- 

 cations with these beautifully printed and liberally illus- 

 trated memoirs, wherein the native asperities of the 

 technical treatise are so smoothed and adorned that they 

 are hardly perceptible. Take, for 

 example .... but comparisons are 

 proverbially odious, and, moreover, 

 the one in mind has been frequently 

 made, with no good result, so let it 

 pass ! 



It is noteworthy that all four 

 treatises give the results of investiga- 

 tions which, although essentially 

 scientific in scope, ha%-e centred 

 around the economic resources of 

 the specified districts. In all cases, 

 also, the prospector and miner, work- 

 ing more or less at haphazard, had 

 made considerable progress in de- 

 veloping the metalliferous deposits 

 before the advent of the geologist, 

 whose function has been to explain 

 the general principles deducible from 

 the discoveries already made, and to 

 indicate the lines along which further 

 exploration may proceed with the 

 best chance of success. This is the 

 proper course, for it is not until the 

 average " practical man " begins to 

 feel the need for professional advice 

 that he is likely to pay much heed to 

 such advice if it be proffered him. 

 All the memoirs, and more especially 

 that on the northern Black Hills, 

 give full descriptions and many 

 principal mine-workings, to which 

 refer. 



First on our list stands the description of the zinc and 

 lead deposits of northern Arkansas, by G. I. Adams, 

 assisted by A. H. Purdue and E. F. Burchard, with 

 a paiaeontological appendix on the correlation of the 

 formations by E. O. Ulrich. Though occurring mainly 

 at a lower stratigraphical position, these metalliferous de- 

 posits appear to be very similar in mode of occurrence 

 and in character of vein-stuff to the lead-ores of the 

 Carboniferous Limestone of the north of England. 



The principal locus of the deposits is in " the Yellville 

 formation," a dolomitic limestone of Ordovician age; but 

 they also range upward, less abundantly, into Lower 

 Carboniferous Limestones. The Silurian system appears to 

 be absent from the district described, and the Devonian 

 is represented only by impersistent sandstone and shale, of 

 which the maximum thickness does not exceed 40 feet. The 

 region has been little disturbed ; igneous rocks are absent ; 

 and the Ordovician rocks still maintain their nearly hori- 

 zontal position. Nevertheless, there has been in some 

 places much differential movement among the strata, prob- 

 ably as the result of compressive forces, whereby the 

 thinner and more brittle beds have been brecciated and the 

 fragments made to rotate or to shear past each other, 

 producing the structure that in this country has been 



termed " crush-conglomerate." These breccias have per- 

 mitted the percolation of the ore-bearing solutions, and 

 are sometimes enriched by metalliferous deposits, though 

 usually only in the vicinity of the nearly vertical fissures 

 which appear to have formed the principal channels of the 

 mineralised waters. It is suggested that the ores repre- 

 sent the concentration of minerals originally disseminated 

 in the country rock, and more especially in the 

 Mississippian (Carboniferous) limestones, this concentration 

 having been effected by waters which, after circulating 

 through the upper belt of weathered rock, have passed 

 downward to the " belt of cementation." 



The ne.xt memoir carries us some 700 miles north-west- 

 ward, to the southern border of Wyoming, and to a 

 geological province of utterly ditterent character. " The 

 Copper Deposits of the Encampment District," by A. C. 

 Spencer, describes a hilly region on the Continental Divide, 

 ranging in altitude from about 6650 feet to 11,007 feet, 

 occupied for the most part by a complex mass of pre- 

 Cambrian rocks, broken into and altered by igneous 

 intrusions, with Mesozoic formations lying upon the flanks 

 of the ancient massif as foot hills and dipping away 

 beneath the surrounding prairie. The pre-Cambrian group 



-Trapper Pi 



illustrations of the 

 we need not further 



includes hornblende-schists derived from bedded volcanic 

 rocks, limestones and shales, quartzite and slate, and a 

 thick conglomerate, with intrusions of quartz-diorites, 

 granites, and gabbros in great variety. The structure of 

 the sedimentary rocks of this group is interpreted as a 

 synclinorium, striking east and west, with its component 

 strata dipping invariably to the south. With respect to 

 the conglomerate, it is noted that though locally almost 

 unchanged from its original condition, it is more frequently 

 metamorphosed, and that this metamorphism, both 

 mechanical and chemical, has often been carried so far 

 that the contained boulders and pebbles have been mashed 

 into disc-like plates, and the rocks, by re-crystallisation, 

 converted into a gneiss the origin of which would be 

 entirely indeterminate except through the study of its 

 gradual passage from the unaltered condition. Certain 

 mineral transformations described in the gabbros are 

 assigned to dynamic pressures insufficient to inaugurate 

 actual crushing, and also unaccompanied by a notable 

 degree of hydration. The copper-ores which constitute 

 the chief mineral wealth of the district occur under diverse 

 conditions, which are carefully described and classified. 

 It is believed that a large part, though not all, of the 

 metalliferous deposits had their original source in 

 the gabbros, of which eighteen samples, representing 

 various phases of the rock, were tested in the laboratory 

 of the survey, and in each case yielded traces of copper. 



NO. T845, "^OL. 7 l] 



