MINERALOGY 45 



hydrate FcgOs • HgO, The third, for which the name limonite 

 is appropriated, is the substance 2Fe203-2H20. It may be 

 noted that E. Posnjak and H. E, Merwin (Amer. Journ. Set., 

 (4), 47, 311, 1 9 19) concluded that the only crystalline forms 

 were the two types of the monohydrate. The occurrence, in a 

 metasomatic iron ore, of an iron silica gel rich in manganese 

 is described by F. Katzer {Cent. Min., 738, 1921), while the 

 formation of iron oolites is discussed by K. Hummel {Metall 

 Erz, 18, 577, 192 1 ). 



The technique of the micro-examination of opaque crystals 

 is treated by M. Frangois and C. Lormand {Bull. Soc. chim. Fr., 

 (4), 29, 356, 1 92 1 ; cf. M. Francois, Compt. Rend., 172, 967, 

 1506, 1921). 



Three papers on the gypsum deposits of the English Mid- 

 lands may be noted. B. Smith {Quartz. Journ. Geol. Soc, 74, 

 174, 191 9) discusses the mode of origin of the brecciated 

 gypsum of Chellaston and concludes that the mineral originated 

 in its present form, and is not derived from anhydrite ; the 

 local occurrences of the latter are likewise primary, evidence 

 of any transformation of the one mineral to the other being 

 practically absent. The fibrous gypsum associated with the 

 breccia is supposed to have been deposited in open fissures, the 

 mineral being of secondary origin. This view is not adopted 

 by W. A. Richardson {Miner. Mag., 19, yy, 1920), who also criti- 

 cises an explanation based on that advanced by S. Taber 

 {Trans. Amer. Inst. Min. Eng., 57, 62, 191 8 ; Journ. Geol., 26, 

 $6, 191 8) in the case of veins of asbestos and similar minerals. 

 According to the latter, the force exerted by the growing 

 crystals is sufficiently great to thrust apart the walls so that 

 the crystals tend to make room for themselves. The former 

 author regards the existence of forces of sufficient magnitude 

 as very doubtful, and suggests that the gypsum veins are 

 produced during the dehydration of the original sediments. 

 Cracks are supposed to be formed during the contraction of a 

 colloidal mass, and in these cracks the gypsum is contem- 

 poraneously deposited from solutions which are possibly trans- 

 formed to the labile condition by release of pressure. In view 

 of the comparatively rapid variation in the solubility of calcium 

 sulphate as the temperature changes and the fact that the 

 influence of pressure on the solubility of salts in water is ex- 

 tremely small, this explanation of the precipitation of the 

 gypsum does not seem very satisfactory. 



In a further paper the same author {Miner. Mag., 19, 196, 

 1 921), on the basis of micro-examination of rock-gypsum, 

 comes to the conclusion that the deposition of the material 

 found in Nottinghamshire was mainly sedimentary^, except in 

 the case of the nodular varieties, which are regarded as con- 



