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treated. We can support the hypothesis of magmatic origin in respect to the 

 iron-ore and sulphide deposits in connection with gabbros, but when huge masses 

 of iron-ore are derived by magmatic differentiation from a granite we feel that the 

 hypothesis is being unduly strained. The magmas from which such opposite 

 poles as granite and iron-ore rock were derived must have had an unusual 

 chemical composition before differentiation, so unusual that most petrologists 

 would not be able to recognise them. In accordance with these extreme views on 

 magmatic segregation even the great pyrite deposits, such as Rio Tinto, are 

 assigned to a magmatic origin. The opinion of Prof. J. W. Gregory, supported 

 by the recent work of Finlayson, that these deposits are much more likely 

 due to metasomatic replacement along zones of faulting and shearing, would, 

 however, be upheld by most mining geologists. 



The illustrations are numerous and excellent. Mr. Truscott is to be con- 

 gratulated on this translation which makes available to English-speaking students 

 the latest and finest Continental work on ore-deposits. We shall look forward 



with pleasure to the continuation of his work. 



G. W. T. 



The Rare Earths : Their Occurrence, Chemistry, and Technology. By 

 S. T Levy, B.A., B.Sc, A-I.C. [Pp. xiv 4- 346, with 11 illustrations.] 

 (London : Edward Arnold, 191 5. Price 10s. 6d, net.) 

 THIS work forms a valuable supplement to our standard text-books in chemistry 

 and mineralogy, and at the same time is of technical importance as regards the 

 trade in incandescent gas-mantles. The prospector will learn from it the variety 

 of minerals on which his attention should be fixed, and tests are given, as in the 

 case of monazite (p. 91), which will be of service to him in dealing with mixed 

 materials in his camp. More emphasis might perhaps be laid on the high specific 

 gravity of many of the valuable materials, a property which leads to their 

 natural concentration. The chapter on monazite sands, however, calls attention 

 to this and affords a good example of the author's appreciation of the practical 

 side of his inquiries. He illustrates an electro-magnetic method of extracting 

 monazite, the mineral becoming held down on a revolving belt if the field is 

 sufficiently strong, and falling off into a receiver when carried beyond reach 

 of the attraction. Good descriptions are given of the deposits that are worked 

 commercially. Mineralogists will note the limited locality of Cerite ; the double 

 discovery of Baddeleyite, the oxide of zirconium, in 1892 in different quarters of 

 the globe ; and the fact (p. 2) that the rare earths generally were held to be 

 restricted to Scandinavia and the Urals down to 1885, when mineralogical 

 investigation became stimulated by technical demands. The historical intro- 

 ductions to each group of earths contain matter of much human interest, such as 

 the capture of Giesecke's specimens, subsequently named Allanite, by an English 

 privateer, and the use of pencils of compressed zirconia in 1867 for the illumination 

 of the Hotel de Ville in Paris. 



The table on p. 113 shows how far we have travelled from the knowledge avail- 

 able in our boyhood. Didymium has disappeared or bifurcated into Praseodymium 

 and Neodymium ; the original term Ytterbia, chosen by Gadolin from the locality 

 Ytterby, and shortened by Ekeberg into Yttria, now reappears, somewhat un- 

 fortunately, as the earth of an element with an atomic weight nearly twice that of 

 Yttrium. This resuscitated name is, however, also threatened (p. 205), and Mr. 

 Levy already speaks of "the old ytterbium," meaning Marignac's element, 

 which was separated from erbium in 1878. Dysprosium dates only from 1906, 

 and Celtium from 19U. 



