82 Reviews — J. H. L. Vogfs Formation of Iron Ores. 



Ill, — On the Forbiation of the Principal Groups of Iron-ork 

 Deposits Occurring in Norway and Sweden. By J. H. L. Vogt. 

 Transactions of the Geological Society in Stockholm, No. 138, 

 Vol. 13, Part 5, May, 1891, pp. 476-536, PI. 8. 

 Om Dannelsen af de vigtigste in norge og Sverige represen- 

 terede crupper af jernmalmforekomster af j. h. l. vogt. 

 Geologiska Foreningens i Stockholm Forhaudlingar, Band XIII. 

 Maj, 1891, s. 476. 



THE author of this valuable communication has, in his work on 

 slags, paid special attention to the conditions under which the 

 silicates and other minerals are developed in magmas of varying 

 composition. He is now extending the principles established by this 

 kind of work to the igneous rocks of volcanic and plutonic origin. 

 In the paper under review he deals especially with the origin of 

 iron-ores of the Ekersund-Taberg type. These he attributes to 

 " magmatic concentration " in strongly basic eruptive rocks ; and in 

 discussing the question of concentration he necessarily deals with 

 matters of general interest to all students of igneous phenomena. 

 Many of the facts referred to by the author have been described by 

 previous observers, but they have never been grouped together so 

 as to illustrate a general theory. Under these circumstances a 

 somewhat extended notice of his paper appears to be thoroughly 

 justifiable. 



The author commences by remarking that ore-deposits may be 

 divided into very well marked groups by taking into consideration 

 the genetic princij)le which has been shown to be of so much value 

 for classificatory purposes in other departments of natural science; 

 and, after a general review of the principal Norwegian occurrences 

 suggests the following classification : — 



I. Ores formed by magmatic concentration in strongly basic igneous rocks. 



II. Ores formed by pneumatolytic processes [i.e. a kind of subterranean fumarole- 

 action ; e.g. tin-stone deposits of Cornwall].! 



III. Ores formed by sedimentation. 



IV. Ores formed by metasomatic processes [e.g. Cleveland ironstone]. 



V. Ores formed by deposition of ferrous carbonate and other minerals in cracks. 

 In the consolidation of igneous magmas the ores and accessory 

 minerals — magnetite, ilmenite, hematite, pyrite, magnetic pyrites, 

 apatite, zircon, spinelle, titanite, perowskite, etc. — are first formed. 

 Then follow the ferro-magnesian silicates — olivine, mica, pj'roxene, 

 amphibole — and lastly the minerals of the felspar group and free 

 quartz. There may be a certain amount of overlapping in the 

 minerals of the last two groups ; but in a general way the oi'der 

 is that indicated above. Now the ore-deposits with which the 

 author is immediately concerned can be accounted for by supposing 

 that the chemical compounds of which they are composed became 

 concentrated by diffusion-processes which are imperfectly under- 

 stood at present. According to this view they would be analogous 

 to the " basic patches " common in many eruptive rocks, 



1 The remarks in square brackets are interpolated by the reviewer. In what 

 follows the author deals exclusively with the ores belonging to the first group. 



