392 



SCIENCE, 



[Vol. VUL, No. 195 



the palpable evidence of the tremeudous efficiency of 

 wind-friction, and realizing the extreme slowness of 

 readjustment of disturbed equilibrium by a slight 

 difference of specific gravity, the other factor, I am 

 compelled to give in my adhesion to the party, very 

 resjjectable in numbers and intelligence, who ascribe 

 the greater efiiciency to the friction of wind. 



So far as the surface-gradients of the ocean are 

 concerned, I must say that I regard them of no 

 significance in this discussion. One has only to 

 turn to Dr. Penk's ' Die schwankungen des meeres- 

 spiegels,' and read the record which he and the 

 authorities he quotes have made, to see that along 

 the coast sea-level is altogether a local phenomenon, 

 and is dependent upon the altitude and jjosition of 

 the neighboring land-masses. Where the shores are 

 lofty mountains, there the water attracted by them 

 rises above the normal ; it also rises on both sides 

 of the Atlantic, and is probably ten or twelve feet 

 lower in the middle than on either side. 



J. S. Newbeeey. 



The genesis of the diamond. 



Prof. Carvill Lewis, in his remarks on ' The gen- 

 esis of the diamond' (Science, viii. p. 345), briefly 

 alludes to the peridotite of Elliott countj% Ky., as 

 ' suggesting interesting possibilities.' My notes 

 (American journal of science, August, 1886, p. 121) 

 on this remarkable eruptive rock are but a brief 

 digest of a re^^ort (Bulletin No. 38, U. S. geological 

 survey, not j'et piiblished) in which its peculiar fea- 

 tm-es are more completely described. If the hjqjoth- 

 esis advanced by Professor Lewis really accords with 

 nature's method of manufacturing this precious 

 gem, it gives to prospectors a most valuable giiide ; 

 and it is well worth while to carefully examine all 

 localities the geological composition and history of 

 which are analogous to that of the South African 

 diamond-fields. 



In Elliott county, Ky., near Isom's mill, six miles 

 south-west of Willard, there are two short dikes of 

 peridotite breaking through the horizontal sand- 

 stones and shales of carboniferous age in such a man- 

 ner as to locally envelop many of their fragments. 

 The slojses in the vicinity are well coA'ered with soil, 

 so that there are but few exposures of either the in- 

 trusive mass or the adjacent strata near the line of 

 contact between them ; and no considerable excava- 

 tions have been made. Nevertheless it is evident 

 that the shales have been distinctly metamorphosed 

 by the peridotite. This is most plainly visible in 

 the enveloped fragments of shale, which are quite 

 numerous in the dike at one exposure near Isom's 

 mill, but elsewhere they are almost or entirely absent. 

 Thus both varieties of peridotite described by Pro- 

 fessor Lewis occur in Kentucky, but the brecciated 

 form has not yet been found to contain diamonds. 



The dark shale, fragments of which are included 

 in the peridotite, may be regarded as composed of 

 sand and clay in varying proportions. The amount 

 of metamorphism experienced by the small frag- 

 ments of shale is very unequal, and by no means 

 proportional to the sizes of the inclusions. One of 

 the earliest and predominant metamori^hic effects is 

 the development of a micaceous mineral in the argil- 

 laceous cement. This development may extend so 

 far as to render the inclusion chiefly micaceous. 

 Each enveloped fragment is surrounded by a narrow 

 zone of colorless mica, the scales of which are fre- 

 quently arranged perpendicular to its surface. An 



advanced stage of metamorphism is marked by the 

 api^earance of very interesting spheroidal bodies with 

 remarkably suggestive properties. They have a high 

 index of refraction, and are pale yellowish to color- 

 less, translucent to almost transparent, and com- 

 pletely isotropic. The diameter of these little glob- 

 ules is generally about .02 of a millimetre, and they 

 are remarkably uniform in size. Rarely this sub- 

 stance appears in irregular grains ; but generally it 

 occurs in a form very suggestive of the diamond, for 

 it resembles a hexoctahedron with curved faces. In 

 general ajapearance it simulates the small translucent 

 crystals of octahedrite in the adjacent peridotite, but 

 their optical properties and action in acids readily 

 distinguish it from that species. They are soluble 

 in concentrated hydrochloric acid, and, when heated 

 to bright redness, they become less translucent and 

 somewhat earthy in appearance ; but the change is 

 not prominent. In the small fragments the globules 

 are usually niimerous, and scattered throughout the 

 scales of clouded mica, but most abundant and least 

 regular in form near the perijDhery of the inclusion, 

 where they sometimes form quite a distinct border 

 just inside the one of colorless mica. In the frag- 

 ments where this jjeculiar isotro^jic substance is most 

 abundant, there is but little well-developed mica. 

 Notwithstanding the fact that some of their proper- 

 ties suggest that they are diamonds more or less jaer- 

 fectly crystallized, their solubility in acid renders 

 such a view untenable. Were they diamonds, they 

 would be of comparatively little value, because of 

 their exceedingly small size. 



The dark shale Avhich is frequently enveloped by 

 the peridotite is somewhat carbonaceoiis, but con- 

 tains a small proportion of carbon as compared with 

 that of the South African diamond-field : for this 

 reason, it appears to me rather improbable that dia- 

 monds will be discovered at the locality in question. 



Some very pretty pyropes, locally sixpposed to be 

 rubies, have been picked up in the soil resulting 

 from the decomposition and disintegration of the 

 jaeridotite, but nothing of greater value has yet been 

 discovered at that place. That the dikes have been 

 l^rospected, and supposed to contain valuable metals, 

 is evidenced not only by slight excavations, but also 

 by the ruins of what ajjpears to have been a structure 

 for reducing ore. Nothing is knowTi in that country , 

 of the history of these ruins, and they may be of 

 considerable antiquity. 



It apjaears to be a significant fact in favor of Pro- 

 fessor Lewis's hypothesis, that the diamonds found 

 in the United States have been discovered where 

 peridotites abound. The chief localities are either 

 in North Carolina and Georgia or in California. Of 

 all the mountain-ranges of this country, the northern 

 portion of the Sierras in California is perhajis the rich- 

 est in serpentine. In cases I have examined, the ser- 

 pentine is derived by alteration from peridotites. In 

 the same region, among older stratified rocks of the au- 

 riferous series, is a black shale or slate which occasion- 

 ally contains a considerable amount of carbonaceous 

 matter; and it is quite possible that the diamonds which 

 have been discovered in the Sierras had their origin 

 along a contact between peridotite and carbonaceous 

 shale. At any rate, the suggestion opens another 

 field for prospectors, and it should be remembered 

 that corundum, with its gems, is also found under 

 similar geologic conditions. J. S. Dillee. 



Petrographic laboratory, U. S. geol. surv., 

 Washington, D.C., Oct. 21. 



