GEOPHYSICAL LABORATORY. 143 



The relations of the kivas to the volcanisiu were examined, and there is a 

 probable connection between the successive changes in the magma and the 

 phases of volcanicity, a feature apparently here recognized for the first time. 

 Such a connection would be of great volcanologic importance, and as there 

 is evidence of its obtaining at other volcanic centers, it will form the subject 

 of future investigation. 



The analyses of these rocks made by the author show that all the earlier 

 analyses, which have been very often cited, are subject to serious analytical 

 errors. The paper closes with a comparison of the Pantellerian lavas with 

 those of other regions. 



(31) Graphical methods in microscopical petrography. Fred. Eugeiio Wright. Am. 

 Jour. Sci. (4), 36, 509-539. 1913. 



Experience in microscopical petrography has shown that the results 

 furnished by graphical means are, as a rule, sufficiently accurate and in 

 accord with the quality of the data of observation. Graphical methods in 

 petrography serve three purposes: (1) to solve certain equations, (2) to 

 represent data of observation, and (3) to picture certain important crystal- 

 lographical and optical relations. In all these cases it is essential: (a) that 

 the graphical means employed represent the relations adequately and as 

 free from distortion as possible; (6) that they are easy of application; and 

 (c) that wherever possible the functions be plotted in such a form that their 

 changes can be represented by straight lines. Eight plates, drawn on these 

 principles, are included, and furnish solutions for the following equations : 



(32) A graphical plot for use in the microscopical determination of the plagioclase feldspars. 



Fred. Eugene Wright. Am. Jour. Sci. (4), 36,540-542. 1913. 



On this plot the changes in the optical properties of the plagioclase feld- 

 spars with chemical composition are indicated by a set of curves, the 

 purpose being to furnish the petrologist, in convenient form and on a single 

 sheet, all the constants essential for the accurate determination of the plagio- 

 clase feldspars in thin rock sections. A new set of curves for the extinction 

 angles on sections showing symmetrical carlsbad-albite twinning is included, 

 the values having been derived from the best available measurements on 

 plagioclase feldspars. 



(33) Magnetite basalt from North Park, Colorado. Henry S. Washington and Esper S. 



Larsen. J. Wash. Acad. Sci., 3, 449-452. 1913. 



This basalt occurs at North Park, forming blocks in a breccia. It is quite 

 unique because of the exceptionally high content in iron ore (55 per cent). 

 A brief description of the results attained with the microscope is given, and 

 a very complete chemical analysis. The basalt differs radically from other 

 igneous rocks high in iron ores in being a lava and not part of an intrusive 

 mass, and also in the very small amount of titanium which it contains. 



