GEOPHYSICAL LABORATORY. 159 



pyrite and bornite, which occur in minor quantities in the enrichment 

 zone, have been synthesized by reactions which may possibly take place 

 there. Secondary pyrite or marcasite — probably both — have been 

 prepared by reactions which may sometimes occur in the enrichment 

 zone. It must be remembered, however, that both these minerals, 

 as well as bornite and chalcopyrite, are by further action of copper 

 sulphate eventually changed to chalcocite. 



When these data are carefully compared with known geologic data, 

 it is hoped they will throw some hght on a number of absorbing ques- 

 tions in geology, e. g. (1) How much of the oxidation of an ore body is 

 accomplished directly by the air and how much by copper sulphate? 



(2) Does enrichment take place above or below the water-level? 



(3) Why is the zone of enrichment so sharply differentiated from the 

 gossan as we find it in nature? 



PUBLICATIONS. 



Brief reviews of the papers published by members of the Laboratory 

 staff during the current year follow: 



(1) Water and volcanic activity. Arthur L. Day and E. S. Shepherd. Smithsonian Report 



for 1913, 275-305. 



Reprinted by permission from the Bulletin of the Geological Society of 

 America, 24, 573-606, 1913. Reviewed in Year Book No. 12, p. 146. 



(2) Crystallization-differentiation in siUcate liquids. N. L. Bowen. Am. J. Sci. (4), 39, 



175-191 (1915). 



Experiments with viscous molten silicates were undertaken in order to 

 ascertain whether the sinking or floating of the crystals which formed could 

 be observed. The sinking of olivine and pyroxene and the floating of tridy- 

 mite were quite readily observed in various mixtures of the system diopside- 

 forsterite-silica. 



With the rate of sinking of the crystals as a basis, rough estimates were made 

 of the viscosity of the melts, and they show, as might be anticipated, a progres- 

 sive increase of viscosity with increase of silica. 



As an illustration, attention is called to the importance of the sinking of 

 olivine crystals in the Palisade diabase sUl, to which J. V. Lewis has already 

 given some consideration. In the opposite sense, contact chilling may, by 

 restricting the sinking of crystals, bring about the formation of a basic contact 

 phase. 



The fact is pointed out that the sinking of crystals can not safely be con- 

 sidered negligible, even in acid magmas occurring in large bodies. 



(3) The accurate measurement of the refractive indices of minute crystal grains under the 



petrographic microscope. Fred. E. Wright. J. Wash. Acad. Sci., 5, 101-107 

 (1915). 



In the exact measurement of refractive indices of minute crystal particles 

 by the inunersion method it is essential: (a) that correctly oriented sections 

 be selected which are normal to at least one principal optic section; ih) that 

 in case oblique illumination be used, only those pencils of light be employed 

 whose direction of propagation is included in the plane normal to the principal 

 optical section — in other words, the metal sliding-stop should be so inserted 



