86 Berieics — J. H. L. Vogt's Formation of Iron Ores. 



rise to the Ekersund deposits, for microscopic examination shows 

 that the minerals have been developed in situ — not mechanically 

 collected — and there are no corroded grains such as might be 

 expected if the second cause had operated. We are therefore limited 

 to the third method. 



The homogeneity of a solution may be destroyed by temperature- 

 differences and by gravity. The influence of the former has been 

 experimentally established by Soret, and follows as a necessary 

 consequence of Van 't Hoff's theorem that osmotic pressure in the 

 case of dilute solutions obeys the laws of gaseous pressure. The 

 influence of the latter has been deduced experimentally by Gouy 

 and Chaperon from the laws of thermo-dynamics. Where solutions 

 become heavier by concentration, the lower part will be more 

 concentrated than the upper part. The difference is slight, and can 

 only be recognized with difficulty when a tube 100 metres high is 

 used. The specific gravity of a molten magma will increase with an 

 increase in the number of molecules of magnetite, ilmenite, magnesia- 

 iron-silicates, pyrite, etc. Hence these molecules will, according to 

 the law of Gouy and Chaperon, be more abundant in the lower than 

 in the upper portion of a magma-basin. 



Differences of temperature operating according to what the 

 reviewer has elsewhere termed Soret's principle will cause the 

 same molecules to accumulate in the colder portions of the same 

 magma-basin. 



Another cause which is believed by the author to be effective in 

 aiding concentration is magnetic attraction. The different iron- 

 compounds when dissolved in silicate-magmas are, doubtless, para- 

 magnetic. Under the influence of the earth's magnetism the 

 molecules may become orientated ; but, so long as they are 

 uniformly distributed throughout the mass, there can be no con- 

 centration due to magnetic attraction. If, however, owing to tem- 

 perature-differences, or to gravity, a local accumulation of magnetic 

 molecules takes place, the magnetic attraction may cause the con- 

 centration to become more and more pronounced. 



The dyke at Huk furnishes an excellent illustration of concen- 

 tration due to differences of temperature. The magma of the ore- 

 deposits of Ekersund and Taberg must, however, have been formed 

 by concentration in the deeper portions of a magma-basin. A certain 

 influence may be ascribed to gravity acting on the specifically 

 heavier molecules ; but it seems hardly probable that the important 

 results which have been observed can be due to this action alone. 

 Magnetic attraction operating constantly and for long periods may, 

 however, when taken in connection with gravity, have given rise to 

 the necessary amount of concentration. 



The liigh specific gravity of the earth may be due to the con- 

 centration of the heavier molecules in the central parts, by the 

 causes above referred to, during the earlier stages of the history of 

 the planet. If so, the ore-deposits under consideration ai-e the genetic 

 equivalents of the originally molten kernel of the globe. 



The intimate relation between the ore-deposits and meteorites has 

 been especially commented upon by Wadsworth. J. J. H. T. 



