January 26, 191 1] 



NATURE 



421 



read, in Berlin Hillebrand announced that the mineral 

 is a mercury-ammonium compound ; it is a mixture of 

 mercury ammonium chloride with some sulphate or 

 oxysulphate. Some interesting photographs illustrate the 

 optical heterogeneity of the mineral. Kleinite is hexagonal 

 in symmetry, but basal sections are only singjj- refracting 

 when heated above 130° ; after cooling very slowly, in 

 process of years, again become biaxial. 



Three of these bulletins (Nos. 397, 408, and 414) state 

 the results of inspections of western mineral fields where 

 mining was once more active than it is now. Mr. F. C. 

 Schrader describes the ore deposits of Mohave County, in 

 north-western Arizona. The country consists of a plateau 

 of pre-Cambrian gneisses covered in places by Cainozoic 

 volcanic rocks, and flanked by Palaeozoic sediments in the 

 valley of the Colorado River. The climate is warm, and 

 with a 5-inch rainfall and high evaporation there is little 

 surface water, and the rocks are oxidised to the depth of 

 usually from 200 to 600 feet. The mines are numerous, 

 but they are all hampered by the difficulties of access and 

 high costs, and so none have been worked very deeply. The 

 outcrops were removed thirty years ago. The mines belong 

 to two main types, one represented in the Cerbat Range, 

 occurring in the pre-Cambrian rocks, and the other, as in 

 the Black Mountain, found in the Cainozoic volcanic forma- 

 tion. The ores in the latter are found only in association 

 with chloritic andesites. The field shows many points of 

 interest, and the results will be watched with interest as 

 the mines go deeper. The plans suggest that some of the 

 ore shoots have been formed from ascending solutions. 

 The evidence available is insufficient to throw much light 

 on general problems, but Mr. Schrader's report will be 

 indispensable in the future development of the field. 



Mr. \V. H. Emmons's reconnaissance on some mining 

 camps in Nevada also deals with small scattered mines of 

 which most of the direct evidence has been lost. Some 

 of them were worked fifty years ago, and mining was most 

 active during the silver boom of the 'eighties. The mines, 

 unlike those in Mohave County, yield a large variety of 

 minerals. The area consists of Palaeozoic sediments, rang- 

 ing from the Cambrian to the Carboniferous, which have 

 been invaded by Cretaceous granodiorites, and covered in 

 places by Miocene rhyolites, andesites, and basalts. One 

 series of mines consists of replacements in what the 

 author calls the " marbleised limestone " around the 

 Cretaceous granodiorites, and a second series occurs with 

 the Cainozoic eruptives, but only in association with the 

 andesites ; the basalts are always barren, and the rhyolites 

 are only productive when near andesite. The chief metals 

 in both series of mines are gold and silver. The gold is 

 relatively more abundant in the older lodes, where it is 

 associated with copper and lead. The mines only occur 

 where the rocks have been leached by hot water, and thus 

 prospectors recognise promising positions by the colour of 

 the weathered rocks. 



Mr. F. L. Ransome has examined Humboldt County, 



Nevada, of which the map prepared by the historic Survey 



of the Fortieth Parallel is still the best. Mining began in 



the district about i860 on ores of antimonial silver with 



stibnite and cinnabar. In Copperwood Canyon small 



\ eins of nickel and cobalt ores occur in an altered andesite 



eside a diorite, probably of Cretaceous age. Mr. Ran- 



ome shows his characteristic insight in the classification 



f the ore bodies and in such illuminating diagrammatic 



-ections as that of the Sheba mine (p. 42); 



The gypsum deposits of California are described in a 

 short memoir by Mr. F. L. Hess. The gypsum is mined 

 for use as plaster and a fertiliser. Some of it occurs in 

 " gypsite, " a material conta^ping grains of gypsum too 

 -mall to be readily discernible to the eye. It is there an 

 fflorescent product, due to the evaporation of water, 

 which has percolated through underlying g}psiferous beds. 

 Some massive deposits formed by the evaporation of 

 shallow lakes and by precipitation in shallow sea water are 

 also of local commercial value. The veins of gypsum, in- 

 cluding both selenite and satinspar, have no intrinsic value, 

 but are worked as the cheapest method of holding mineral 

 leases on land which may \ield oil. Sufficient work is 

 done on the gypsum to maintain the lease without the 

 expense of boring for oil, and thus dodging the law bv 

 using one mineral to maintain an unfair claim to another. 

 The valuation of coal lands is a problem which has long 



NO. 2152, VOL. 85] 



taxed the ingenuity of experts on mining law. In Bulletin 

 424 Mr. Fisher contributes to the discussion a summary 

 of the depth and minimum thickness of coal seams worked, 

 in various countries. The deepest coal mining recorded is 

 from 3937 feet, in Belgium ; the deepest in Britain is at 

 3483 feet, in Rams Mine, Pendelton ; and depths of more 

 than 3000 feet have been reached in France and Germany- 

 Forty years ago a British Coal Commission recognised 

 that mining would reach a depth of 4000 feet, but such, 

 is the wealth in fuel of the United States that coal bek>w 

 3000 feet is still disregarded in valuation. The United- 

 States, moreover, has not yet been driven to work such, 

 thin coals as are wrought in England and Belgium. The 

 thinnest English seams worked at present independently 

 are a cannel coal of 8 inches and ordinary coal 10 inches 

 thick. Seams 12 inches thick are worked in Belgium and 

 Scotland, where beds of less than 2 feet thick are worked 

 extensively. 



The red iron ores in the Silurian rocks of Alabama,, 

 described in Bulletin 400, are second in importance in the 

 United States only to those of the Lake Superior dlstrict. 

 They are low-grade ores, but being near fluxes and fuel are 

 cheaply w<M-k^. The Clinton ores have generally been 

 regarded as a residual deposit due to concentration of iron 

 oxide by solution of a ferruginous limestone. This view 

 has been based upon the belief, due to Porter and I. C^ 

 Russell, that the ironstones pass below into normal lime- 

 stone. This view has been accepted by many later 

 economic geologists, but is rejected by Eckel, as the ore- 

 is already being mined far from the outcrop, and has been 

 found in New York in bores ten to fifteen miles from the 

 outcrop, and nearly 1000 feet below the surface. The ore 

 is often oolitic and contains many marine fossils which 

 have been altered into iron oxide, but that this change 

 happened during the deposition of the rock is indicated 

 by several facts. Thus many of the oolitic grains contain 

 a nucleus of quartz grains surrounded by concentric layers 

 of iron ore, which is covered by carbonate of lime. A 

 fuller account and figures of the microscopic structure of 

 the ores would have been useful. .As the oolitic grains 

 have been cemented by iron oxide, some replacement 

 appears to have taken place after the formation of the- 

 bed. Mr. Eckel, however, produces weighty evidence in 

 support of his view that the ore is mainly of contemporary 

 origin, though recent work shows that other American 

 geologists reject this explanation, and regard the estimates 

 based on it as exaggerated. 



The brown ores of Alabama are admitted by Mr. Eckel 

 to be epigenetic ; they are interbedded with Cambro- 

 Ordovician, Cretaceous, and Cainozoic rocks, but are all 

 of Cainozoic formation. 



Mr. Maddren's report on some Yukon placer deposits 

 shows that the gold has been derived from lodes formed 

 by the intrusions of acid rocks in Mesozoic or Lower 

 Cainozoic times. The gold is usually coarse, but its con- 

 centration has been slow, because the cold acts as a 

 cementing agent, and the erosion of the frozen ground is 

 very slow. The report gives some interesting information 

 as to the relative extent of Glacial and post-Glacial 

 denudation in some Alaskan vallevs. 



J. W. G. 



ON THE SENSIBILITY OF THE EYE TO 



VARIATIONS OF WAVE-LENGTH IN THE 



YELLOW REGION OF THE SPECTRUM.' 



"TJR. EDRIDGE-GREEN' has introduced a method of 

 "^ classifying colour-vision by determining the number 

 of separate parts or divisions in the spectrum within each 

 of which the observer can perceive no colour difference. 

 Movable screens are provided in the focal plane of the 

 spectroscopic telescope, by which the part admitted to the 

 eye is limited and the limits measured in terms of wave- 

 length. Beginning at the extreme visible red, more and 

 more of the spectrum is admitted until a change of colour 

 (not merely of brightness) is just perceptible. This gives- 

 the first division. The second division starts from the 

 place just determined, and is limited in the direction oV 



1 .A.bs»ract of a paper read before the Royal Society on December S, 1910, 

 by Lord Rayleigh, O.M., F.R.S. 

 3 Roy. Soc Proc., B, 1910, vol. bucxii., p. 458, and earlier writings. 



