Geology and Natural History, 83 



A careful study of the morphology of the region is given by 

 the author, the quaternary geology by him and Rekstad, and 

 the work is concluded with an account of the silver ore veins of 

 Svenningdalen. 



A resume in German is given at the end. The work through- 

 out is full of the suggestiveness in ideas and thoroughness in 

 details that are characteristic of the author. l. v. p. 



9. Classijication of Igneous Bocks. — E. von Federov, in an 

 article published in the Journal of the Mineralogical Society of 

 St. Petersburg in 1900, page 395, gives a method for the classifi- 

 cation of igneous rocks in which he endeavors to show that it is 

 possible to divide all of the eruptive rocks into types and classes 

 by assigning to them simple symbols like those used in crystal- 

 lography. 



In considering the general chemical composition of rocks in the 

 order of the oxides, 1, R 2 ; 2, RO ; 3, R 2 3 ; 4, R0 2 , it be- 

 comes evident that all of the ordinary rock-making minerals may 

 be expressed by symbols of four numbers. Thus, quartz (0001), 

 corundum, hematite, etc. (0010), spinel, magnetite (0120), olivine 

 (0201), diopside and actinolite (01 01), garnet (0323), biotite (1223), 

 phlogopite (1313), anorthite (0122), orthoclase and albite (1013), 

 leucite and aegirite (1012), nephelite (8089). 



The reckoning of the different compositions is based on a reg- 

 ular tetrahedron whose summits are the points (1000), (0100), 

 (0010), (0001); the center of the faces are the points (01 11), (1011), 

 (1101), and (1110); the center of the edges (1100), (1010), (1001), 

 (0110), (0101), (0011), whilst the center of the tetrahedron is 

 (1111). 



Using these points the tetrahedron can be divided into 24 

 sphenoids, to whose centers there correspond 24 typical composi- 

 tions. Of these fundamental types of chemical composition only 

 four are represented, those having the symbols (1423), No. i, 

 (1324) No. ii, (1234) No. in, and (2134) No. iv. No. 1 is repre- 

 sented by the periodotites, No. 2 by rocks of the gabbro group, 

 No. 3 by rocks of the diorite and diabase groups, and No. 4 by 

 rocks of the granite and syenite groups. 



The division of the types into classes is done by dividing the 

 sphenoids into 24 sphenoids of the third order. By the aid of 

 the principles of zonal crystallography the author explains the 

 process to be followed to obtain the chemical composition when 

 the indices corresponding to the third order are given, and how to 

 determine if this composition is a typical one (represented by an 

 interior point of the sphenoid), one intermediate between the 

 classes, one of transition between the great types, or an extreme 

 rock. 



The author shows how these may be determined by graphic 

 methods and makes a number of applications of his system. The 

 main body of the text is in Russian, but a short resume in French 

 permits the salient features of the article to be understood by 

 those who are not versed in that language. l. v. p. 



