RECENT ADVANCES IN SCIENCE 41 



F. N. Guild (ibid. pp. 297-353, 191 7) has made an elaborate 

 micro-study of silver ores, and has critically examined the 

 criteria used in the diagnosis of the minerals concerned. In 

 the paper, which contains a very fine series of photomicro- 

 graphs, numerous deductions are made concerning the order 

 of deposition of the various minerals. The frequent occurrence 

 of graphic structure is interesting, and is ascribed to two 

 causes, firstly, replacement, as in the " intergrowths " of 

 stromeyerite (CuAgS) with chalcopyrite and bornite, and 

 secondly, simultaneous deposition followed by segregation, as 

 in the intergrowths of stromeyerite with chalcocite, argentite, 

 and galena. The peculiar structure which has been held to 

 indicate heterogeneity in stromeyerite is shown to be due to 

 the presence of chalcocite as an impurity, stromeyerite being 

 held to be a true double sulphide of copper and silver. By 

 similar methods, E. S. Bastin (ibid. pp. 219-36) has examined 

 the silver ores of Cobalt, Ontario. By analogy from quanti- 

 tative synthetic experiments by C. Palmer on the precipitation 

 of silica from solution by certain arsensides (ibid. pp. 207-18, 

 191 7), it is concluded that the replacements in the ores of this 

 region can be explained by iron-bearing solutions of silver 

 sulphate, probably containing free acid in addition. 



These methods have also been utilised by L. P. Teas (Bull. 

 Amer. Inst. Min. Eng., pp. 191 7-31, 191 7) in an examination 

 of the relationships of sphalerite, and one interesting conclusion 

 reached is that chalcopyrite is present when the sphalerite 

 has been deposited from ascending juvenile waters, but absent 

 when meteoric solutions constitute the origin. 



An elaborate paper by A. B. Dobrovolski (Archiv. Ketni, 

 Min. 0. Geol., 6, pp. 1-53, 1916) gives a critical summary of 

 the work which has been done on the crystallography of ice. 

 The form development, habit, and twinning of the natural 

 mineral are discussed, and the conclusion reached that it 

 belongs to the tourmaline class (ditrigonal pyramidal) of the 

 hexagonal system, thus confirming Nordenskiold's suggestion 

 that the mineral is hemimorphous. The axial ratio, however, 

 is still very much in doubt. The interesting question as to 

 whether the unstable modifications, described by Tammann, 

 Wallerent, and others, and capable of existence at atmospheric 

 pressure, occur in Nature, is also left in doubt. 



Meteorites. — The most interesting paper on this subject 



