40 SCIENCE PROGRESS 



dolomite, with a very small proportion of interstitial clay. 

 Chemical analysis shows 98*5 per cent, of calcium-magnesium 

 carbonate and 1*5 per cent, alumina and iron oxide. The 

 plasticity is supposed to be due to the fact that the rhomb 

 faces of adjacent crystals are separated by a very thin film 

 of water which allows the faces to slip over each other without 

 breaking the actual contact. Another peculiar clay is de- 

 scribed by E. W. Hilgard (Proc. Nat. Acad. Sci. 2, pp. 8-12, 

 1 91 6). Chemically, it is a hydrated magnesium calcium 

 silicate containing a small proportion of sodium salts and 

 about 1 5 per cent, of absorbed water. The material is mainly 

 colloidal except for some small grains of crystalline carbonates. 



A number of papers dealing with the mineralogy of sul- 

 phides have appeared recently. R. C. Wells and B. S. Butler 

 (Journ. Wash. Acad. Sci. 7, pp. 596-9, 191 7) describe a new 

 mineral, the chemical composition of which is sulphide of 

 tungsten (WS 2 ), and to which they have given the name " tung- 

 stenite." The mineral is black and opaque, has a high specific 

 gravity, and is not attacked by mineral acids. Another new 

 sulphide belonging to the jamesonite group is described by 

 E. V. Shannon (Amer. Journ. Sci. 45, pp. 66-70, 191 8). It is 

 probably orthorhombic, has the formula 5PbS,2Sb 2 S 3 , and 

 has been named " mullanite." An occurrence of the rare 

 mineral chalmersite (CuFe 2 S 3 ) as a prominent constituent of 

 some ore-bodies in Alaska is described by B. L. Johnson (Econ. 

 Geol. 12, pp. 519-25, 191 7). 



In recent years great advances have been made in the 

 examination of opaque minerals by methods analogous to 

 those used in metallography, and much valuable information 

 regarding the structure and genesis of ore deposits has been 

 obtained by these means. The methods of identification have 

 been admirably described by J. Murdoch (The Microscopical 

 Determination of the Opaque Minerals, New York, 1916), and in 

 a recent paper W. L. Whitehead (Econ. Geol. 12, pp. 697-716, 

 191 7) gives a fairly exhaustive treatment of the technique of 

 " mineragraphy." The methods of grinding and polishing, of 

 micro-examination and photomicrography are discussed, and 

 Murdoch's tables are supplemented by several new reactions 

 for certain minerals. The methods of " light-etching " which 

 the author recommends for the discrimination of silver minerals 

 are described in detail. 



