September 15, 1911] 



SCIENCE 



349 



mineralogical composition of granites and 

 gneisses has just been issued. This tireless 

 worker in the fields of naineralogy and petrog- 

 raphy presents in this his latest work a very 

 thorough study of his subject, and offers a 

 large mass of materials derived from the best 

 sources and supplemented by the results of his 

 own investigations. The details are so 

 grouped as to be easily utilized by those who 

 may consult the volume. The importance of 

 a correct determination of the composition of 

 granite rocks for an understanding of the 

 genesis of mineralogical forms in general does 

 not need to be emphasized. 



The author begins his exposition with an 

 examination and description of the diilerent 

 methods used in determining the quantitative 

 composition of rocks (pp. ll-TS), a thorough 

 understanding of the various methods being 

 essential for the appreciation of their respect- 

 ive value in controlling the results arrived at 

 by any one of them. The purely chemical 

 methods are first described, it being shown 

 that the investigation of the mineralogical 

 composition of granite should begin with a 

 determination of the quantity of magnesia 

 present in the biotite or biotitic granite. A 

 table on page 14 shows the diiierences in rela- 

 tive percentage of the constituents of ortho- 

 clase, albite and anorthite, as conditioned by 

 the later and earlier computations of the 

 atomic weight of the elements; the first col- 

 umn gives the figures arrived at or according 

 to the standard of 1907, and the second and 

 third columns the figures according to earlier 

 standards — the third column referring to the 

 early sixties. 



The method followed by Haughton in his 

 analyses of Leinster granite — wherein he rec- 

 ognized as the three principal constituents, 

 feldspar, margarodite and quartz — and that 

 followed by Sartorius von Waltershausen in 

 the investigation of basalts, are given at con- 

 siderable length (pp. 17-33). The author 

 then explains the method of determination 

 by the use of reagents and finally proceeds to 

 a consideration of the purely mechanical 



methods. He gives the preference to that used 

 by Delesse — as early as 1848 — and modified 

 and improved by later investigators. 



As many of the results secured by Pro- 

 fessor Tchirwinsky were obtained by the use 

 of this method, we give briefly his description 

 of it, as used by him. Upon the plate to be 

 examined a network of points is made with 

 ink. The interval between the single points 

 and the width of the network are determined 

 by the magnification to be used. The point 

 at the upper left-hand corner is first brought 

 to the center of the cross-threads over the ob- 

 ject glass of a microscope, the instrument 

 resting in a horizontal position in a Winkel 

 or Leitz apparatus for microphotography. In 

 place of the ground-glass disk, a disk of trans- 

 parent glass is inserted in the apparatus, and 

 a piece of tracing paper is attached to the 

 glass with wax. Under artificial illumination 

 of the apparatus in a dark room, a sharp, 

 clear picture of that portion of the plate sur- 

 rounding the point is projected on the paper 

 and is traced there with a pencil. Similar 

 minerals are designated by given letters and 

 are afterwards cut out. The same process is 

 followed with all the points. 



More than 400 pages (pp. 77^95) are de- 

 voted to tables, arranged geographically, ex- 

 hibiting the results of analyses of granites 

 and gneisses. The various data are examined 

 critically, and are supplemented by the results 

 of numerous tests made by the author accord- 

 ing to the Delesse method. These are given 

 at great length in the case of each plate 

 tested, with the average value. The material 

 here assembled is very valuable and of great 

 interest to the petrographer. 



This section is followed by one presenting 

 the conclusions drawn by the author from the 

 material he has brought together (pp. 501- 

 659). Here the manifold characteristics and 

 the average chemical composition of the sev- 

 eral minerals that appear in granite are stud- 

 ied, as for example the feldspars, biotite, 

 hornblende, pyroxene, quartz, etc., and the 

 quantitative chemical and mineralogical 

 composition of granite, pegmatite-granite, 



