PETROLOGICAL ABSTRACTS AND REVIEWS 711 



A modification of Becke's method for determining 2V, here given, greatly 

 simplifies the process. The isogyre is first placed parallel to the horizontal 

 cross-hair of the microscope, and the position of the melatope is determined by 

 two angles, one measured in azimuth from the vertical cross-hair to the melatope 

 by rotating the stage, the other measured from the center by means of a grad- 

 uated eyepiece and any method similar to that employing a Schwarzmann's 

 axial angle scale. The position of the melatope {A) is marked on a stereo- 

 graphic net and a great circle is drawn through it and the ends of the horizontal 

 line. So far the method agrees with that of Becke. Change the conoscope into 

 an orthoscope and rotate the stage imtil the section is at extinction and read 

 the angle through which the stage was rotated. This locates one of the vibra- 

 tion directions which is now drawn at the proper angle in the projection. Lo- 

 cate, on the great circle previously drawn, the other melatope by means of an 

 angle equal to that between the first melatope and the Une representing the 

 vibration direction. Measure 2F in the projection. In the older methods it 

 was necessary to observe a second point on the isogyre, which was difiicult. 

 In the Tsuboi method only the position of the melatope is needed. Further, 

 there is no need of using the refractive index in the new method. In Becke's 

 original, five great circles were necessary, and in Wright's modification, four. 

 In Tsuboi's, only one great circle is drawn, and one straight line. 



Tyrrell, G. W. "A Contribution to the Petrography of Ben- 

 guella, Based on a Rock Collection Made by Professor J. W. 

 Gregory," Trans. Roy. Soc. Edinburgh, LI (1916), 537-59, pi. i. 



Benguella is one of three provinces of the Portuguese West African colony 

 of Angola. Chemical analyses and complete descriptions are given of the rocks, 

 which are granite, charnockite, dellenite, nephehte-sodalite-syenite, akerite, 

 shonkinite, solvsbergite, ouachitite, and various basic intrusives. The two 

 "charnockites" of Table I are 227' (new form of reviewer's system) or monzo- 

 tonalites (granodiorites in limited sense), while the type charnockite from India 

 is 226', or typical granite. The hornblende-hyperites belong to 3312, as they 

 shovdd. The granite of Table II is 216', typical granite; the granodiorite is 

 227', granodiorite in sense usually used but better monzo-tonalite. The two 

 dellenites computed in Table IV are 227' for the Angola rock, and 227" for the 

 one from Sweden. That is, the former is the extrusive equivalent of a grano- 

 diorite, while the latter is the extrusive equivalent of a quartz-monzonite. The 

 shonkinite of Table V is 2 113, which is not according to definition of shonkinite 

 as originally give by Pirsson, for in that the dark constituent must form 

 more than half the rock, consequently it must be in Class 3, as actually is 

 the type Montana specimen given in the same table. The Angola rock falls 

 into the group with pulaskites although the feldspathoid in the latter rock was 

 given by Williams as nephelite or its decomposition product analcite. 



