PETROGRAPHICAL ABSTRACTS AND REVIEWS 183 



pages are devoted to the "Illumination of the Object." An explanation 

 of the "white-line effect" (Becke's line) is given for parallel light where 

 the contact plane of the two minerals in question is at various inclinations. 



W. T. SCHALLER 



Grayson, H. J. "Modern Improvements in Rock Section Cutting 

 -Apparatus," Proc. Roy. Soc. Victoria, XXIII (1910), 65-81. 

 Pis. 4. 



Describes an apparatus, constructed for the University of Melbourne, 

 with which the writer is able to slice, grind, and mount thin sections of 

 about an inch in diameter and of a thickness of less than 0.001 inch, 

 from rocks of the hardness of granite, in not more than ten minutes. 

 Using two cuts with a diamond saw for each slide, the cost per section 

 is about one shilling. 



A mechanical device for doing the rough grinding would be an im- 

 provement. With a number of laps running simultaneously, the greater 

 length of time required for each section would be no drawback, and there 

 would be a considerable reduction in cost since it would not be necessary 

 to use diamond dust. 



Albert Johannsen 



Grout, Frank F. "The Composition of Some Minnesota Rocks 

 and Minerals," Science, XXXII (1910), 312-15. 



A preliminary statement regarding the composition of certain Minne- 

 sota rocks. There are given analyses of seven rocks and fourteen 

 minerals. 



Two or three types of granite occur in laccoliths of considerable size 

 in the Keewatin schists and are considered by the author as probably of 

 that age. These granites are intersected by diabase, quartz diabase, 

 and quartz porphyry dikes, and there occur a few masses of gabbro. 

 Most of the Minnesota effusive rocks belong to three types of diabase 

 which, chemically, are classed as Hessose, Bandose, and Auvergnose. 



The country rock was tested for copper. The common theory of 

 the origin of the Lake Superior copper deposits is that of lateral secretion 

 from the diabases. In the present tests it was found that copper occurs 

 in all the main types of rock, and, so far as could be judged from the ten 

 samples tested, the fresher the rock, the larger the amount of copper. 

 It varied in amount from 0.029 to 0.012 per cent. 



Albert Johannsen 



