382 SCIENCE PROGRESS 



genesis of the ores as indicated by recent exploration (Trans. 

 Amer. Inst. Min. Eng., 191 8, 59, 27-67). They leave uncertain 

 the nature of the differentiation by which the micropegmatite 

 and norite members have been produced ; but believe that 

 the sulphides were carried down by the norite, remained in 

 solution while the norite consolidated, and were concentrated 

 in association with an acid extract which solidified as granite. 

 The presence of mineralisers in these solutions enabled them 

 to replace the wall rocks to some extent. The dominant factor 

 controlling the deposition of the ore is thus magmatic segre- 

 gation in situ ; but the ore belongs to the latest stage in the 

 history of the magma. This view has a considerable resem- 

 blance to that put forward by Goodchild (see Science Pro- 

 gress, 13, 1919, 376). 



An occurrence of chalcopyrite and bornite in syenite and 

 syenite-pegmatite dykes (La Fleur Mountain, Washington) 

 may be added to the list of the magmatic sulphides, according 

 to an investigation by McLaughlin (Econ. GeoL, 19 19, 14, 403- 

 10). These minerals are later than the normal rock minerals, 

 a sequence which is found to hold with most occurrences of 

 magmatic sulphides. 



An important paper by Billingsley and Grimes (Trans. 

 Amer. Inst. Min. Eng., 191 8, 58, 284-368) seeks to compare 

 the origin of the various ore deposits associated with the great 

 Boulder batholith of Montana, in which Butte, the most im- 

 portant copper region of the world, is situated. It was found 

 that the ores are associated with intrusive rather than extru- 

 sive igneous action, the largest number of deposits being con- 

 nected with the period of greatest intrusion. Successive 

 magmatic differentiates furnish increasing proportions of vein- 

 forming solutions. The sequence of ore deposition follows a 

 definite order in any igneous phase : first, contact and border ; 

 second, internal segregation ; and third, fissure and fault 

 veins filled from deep-seated sources. A number of other 

 interesting generalisations for the student of ore deposition 

 in relation to igneous action are given, which it is impossible to 

 quote here. 



The Biwabik iron-bearing formation (taconite) of the Mesabi 

 Range, Minnesota, is believed by Grout (Econ. GeoL, 1919, 14, 

 452-64) to have been deposited in shallow water mainly by 

 precipitation through organic processes, resulting in the pro- 

 duction of lean, ferruginous cherts, with more or less iron- 

 bearing carbonates, ferric oxide, and greenalite. Alternating 

 with periods of precipitation came periods of solution, leach- 

 ing, oxidation, and wave action, producing conglomerate and 

 granular rocks much enriched in iron, and probably some 

 layers of pure ferric oxide. Deep burial under slates developed 



