abstracts: mineralogy 503 



the first time. Such a connection would be of great volcanologic im- 

 portance, 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 Pantel- 

 lerian lavas with those of other regions. H. S. W. 



PETROLOGY. — The index ellipsoid (optical indicatrix) in petrographic 

 microscope work. ' Fred. Eugene Wright. Am. Jour. Sci. (4), 35: 

 133-138. 1913. 

 In this paper the importance of presenting the subject of microscop- 

 ical petrography consistently from the viewpoint of the index ellipsoid 

 aS' applied to wave-front normals is emphasized. The various optical 

 properties employed in practical petrographic microscope work can be 

 best described and explained systematically, by means of the index 

 ellipsoid. The use of the so-called "axes of elasticity," a, b, C, or X, Y, 

 Z, in this connection is confusing and only adds to the difficulties 

 encountered by the observer in mastering the subject. They should 

 accordingly be abandoned and the French usage of naming the princi- 

 pal axes of the index ellipsoid, a, /3, y (or Up, nm, ng) adopted. This 

 applies in particular to the different modes now in vogue for expressing 

 extinction angles. For a given crystal face an extinction angle is simply 

 the angle between a definite crystallographic direction on the face and 

 one of the axes, a' or y', of its optic ellipse, and this fact should be indi- 

 cated in the expression for the extinction angle. To introduce "axes 

 of elasticity" (a', c', or X', Z') in this connection is not only needless 

 but less direct, as it introduces entirely new conceptions which, experi- 

 ence has shown, only tend to bewilder the student. F. E. W. 



MINERALOGY. — Two varieties of calciovolborthite{?) from Eastern Utah. 



W. F. HiLLEBRAND and H. E. Merwin. Am. Jour. Sci. (4), 35: 



441^45. 1913. 

 From chemical studies, two minerals from Paradox Valley, Colorado, 

 are considered to be varieties of calciovolborthite. In the absence of 

 optical data concerning the original mineral, the following optical 

 properties, determined from one of these varieties, .are assigned to 

 calciovolborthite. Color, yellow-green, with no distince pleochroism; 

 biaxial, with strong inclined dispersion; optically negative for blue and 

 positive for red; a^g, = 2.01, (8Na = 2.05, 7xa = 2.10. The optical 

 properties give evidence of monoclinic symmetry. W. F. H. 



