172 Scientific Intelligence. 



tion. In this they are in striking contrast to the generality of 

 geological maps, such for instance as the new geological atlas of 

 Germany, issued at Gotha under the supervision of Prof. 

 Lepsius, whose glaring colors and sharp controsts offend the eye. 

 In only one point does it appear that the Livingston map could 

 have been improved. In the areal geology sheet the tone of 

 the color selected for Archaean is too dark, making it difficult to 

 see the contour lines and to study the geology in connection with 

 the relief form. The same may be said of the areas of basic ande- 

 sitic breccia. On the other hand the tone of the same color em- 

 ployed for the corresponding areas on the economic sheet is all 

 that could be desired, in this respect. 



The use of yellow instead of the conventional tones of red to 

 designate the igneous rocks will appear novel to many, but it must 

 be admitted that it adds to the artistic beauty of the map and 

 defines the areas with great clearness. l. v. p. 



2. Relation of double refraction to Soda in Hornblende ; by 

 Alfred C. Lane. (Communicated). — I find that the following 

 law fits not only my own observations on hornblende, but also 

 such few indications as I can find in the literature, — 



n — 90/1 V (0-012 — 5) 



where n is the amount of soda contained, and b the refraction of 

 the orthopinacoidal (100) section ; b to be considered negative, as 

 is natural, whenever the refraction for the vertical axis is less than 

 for the lateral. That this double refraction should be dependent 

 on the amount of soda is rendered less surprising by the fact that 

 with the increase of soda there is generally also a progressive 

 absorption of the red end of the spectrum in the same section. 



If the amphiboles contained but two molecules, one containing 

 soda and the other not, a law of the above form must theoretically 

 be true as a first approximation,* but I am rather surprised that 

 it seems to apply so widely as it does in so complex a group. It 

 is to be remembered, however, that for any two molecules having 

 the same axial plane there will be a couple of sections at right 

 angles to the axial plane which will have the same double refrac- 

 tion in each molecule, and therefore will not vary in double re- 

 fraction in isomorphous mixtures of the two. It is probably true 

 therefore that for the commoner and more important non-sodifer- 

 ous molecules the orthopinacoidal double refractions are nearly 

 the same. This is not true, however, for the brown Bohemian 

 hornblende described by Levy and Lacroix, and this is the only 

 case that I know of where the law can be said definitely to fail. 

 But observations are few, and the main object of this communica- 

 tion is to call the attention of fellow-workers to the subject, and 

 urge upon all those who make hornblende analyses to observe this 

 double refraction. Or, if they will exchange material with the 

 writer he will be very much obliged. 



*Pockels, Neues Jahrbuch, 1893, Beilage Band viii, p. 135. . 



