MINERALOGY 43 



of some of the crystals to these radiations was also examined, 

 thin plates approximately 5 mm. in thickness being used. 

 Both zinc blende and wurtzite were highly transparent, but 

 rutile, owing to the great amount of reflection, was only 

 slightly so. 



The thermal expansions (linear) of pyrrhotite and magnetite 

 have been examined by P. Chevenard (Compt. Rend,, 172,320, 

 1 921). For the former mineral, the curve showing the linear 

 expansion increases to a maximum at 320° C, and then remains 

 parallel to the horizontal axis, while in the case of the latter, 

 the corresponding curve shows a maximum at 570° C, the 

 coefficient of expansion diminishing above this temperature. 

 Pyrrhotite is therefore assumed to have a polymorphic trans- 

 formation at 320° C. and magnetite at 570° C, the high tem- 

 perature form in the latter instance being less dense than the 

 low temperature form. 



The value of physical data, such as density, elasticity con- 

 stants, electrical conductivity, in geological investigations is 

 discussed by R. Ambronn (Gluckauf, 57, 481, 1921 ; Chem. 

 Zentr., 3, 286, 1921). The classification of magmatic pheno- 

 mena, on the basis of physical chemical considerations, is treated 

 by P. Niggli (Naturwiss., 9, 463, 192 1). A petrographic investi- 

 gation of the formation of pseudomorphs of chlorite, sericite, and 

 quartz after felspar is described by H. Laubmann [Neues 

 Jahrb. Min., 1, 15, 1921). A discussion, from the chemical 

 point of view, of the origin of certain lamprophyres is given 

 by P. J. Beger {Zeit. Kryst., 56, 417, 1922). 



In a paper on the " paragenesis " of a-quartz, A. Johnsen 

 {Sitzber. Bayr. Akad., 321, 1920) endeavours to calculate the 

 conditions under which a particular specimen developed, on the 

 basis of the ratio of liquid to gaseous carbon dioxide in the 

 inclusions, used in conjunction with already known physical 

 chemical data. The use of quartz as a " geological thermo- 

 meter " is discussed by G. Miigge {Cent. Min., 609, 641, 1921). 



The conditions under which fluid inclusions are formed in 

 minerals is discussed by R. Nacken {Cent. Min., 35, 1921). 

 The basis of the discussion is the temperature-pressure diagrams 

 of the system water-carbon dioxide, but the author concludes 

 that in general no satisfactory determination of the pressure 

 of formation can be made, although in the case of some water 

 inclusions it is possible to fix the temperature of formation of 

 the host crystal within narrow limits. 



From an investigation of the properties of chalcedony and 

 black flint, E. W. Washburn and L. Navias {Proc. Nat. Acad. 

 Sci., 8, I, 1922) conclude that both these minerals are essentially 

 composed of quartz. This deduction is verified by a study of 

 the inversion temperatures and the X-ray spectra. 



