PETROLOGICAL ABSTRACTS AND REVIEWS 479 



tion of the condition of hydrogen in minerals is developed. The author 

 advocates the weighing of the air-dry powder and determination of mois- 

 ture rather than the weighing of the perfectly dried sample, as the latter 

 takes up water on the unavoidable exposure to the air during weighing, etc. 



A considerable addition is made to the chapter on the determination of 

 titanium, with the view to minimizing the error due to the bleaching effect 

 of alkali salts on the color produced by hydrogen peroxide. 



A new indirect method for determining fluorine, devised by Steiger 

 and modified by Merwin, is introduced. This method is based on the fact 

 that fluorine has a powerful bleaching effect on the yellow color resulting 

 from the oxidation of a titanium solution by means of hydrogen peroxide. 

 Steiger's and Merwin's diagrams are introduced. 



A namber of alterations of minor importance have also been made. 



Albert D. Brokaw 



JOHANNSEN, Albert, "Somc Simple Improvements for a Petro- 

 graphical Microscope," Am. Jour. Sci., 4th ser., 1910, XXTX, 

 435-38- Figs. 4. 

 The writer describes (i) a rotating upper nicol in which the annoying 

 reflection of light from the surface is overcome; (2) a permanently attached 

 combination wedge, combining a gypsum plate and a combination quartz- 

 mica wedge in a metal casing. An indicator shows the order of the inter- 

 ference color appearing in the field; (3) a rotating lower nicol for observing 

 very slight changes of pleochroism; (4) improvements to the Hirschwald 

 stage. 



Albert D. Brokaw 



Lacroix, a. " Sur I'existence a la Cote d'lvoire d'une serie petro- 

 graphique comparable a celle de la charnockite, Comptes Rendus 

 de VAcademie des Sciences, 1910, CL, 18-22. 



The rocks described have a vast development in the western part of 

 the Gold Coast territory. They range from hypersthene granite, almost 

 wholly composed of quartz and feldspars, to a norite free from quartz 

 and containing at least 50 per cent of hypersthene. 



Four analyses are given. These illustrate the following chemical 

 characteristics; relatively high iron and magnesia, varying inversely as 

 SiOa, (71 .80-54.33); slight variation in soda (3.52-4.35), rapid decrease 

 in potash (4.1 1-0.59) ^^ silica decreases; relatively small variation in lime 



