282 Reviews — Origin of the Iron Ores at Kiruna. 



American resources, actual and potential, but also a useful summary 

 of foreign sources, accounts of the tests for potash, and processes 

 of extraction, and finally a very full bibliography (uj) to and including 

 1918) of the whole subject. 



Arthur Holmes, 



Origin oi' the Iron Ores at Kiruna. By R. A. Daly. 

 Vetenskapliga och prakti&ka Undersokninsar i Lappland, 

 Geology of the Kinnia District, part v. pp. 31, with 4 figures. 

 Stockholm, 1915. 

 rpHE magnetite ore-bodies of Kirunavaara-Luossavaara are in 

 -*- some respects the finest known examples of the differentiation 

 of oxides from an igneous magma, and there has naturally been a 

 good deal of discussion as to the mechanism by which this has been 

 brought about. Some writers consider the porphyries associated 

 with the ores to be lava-flows, but Daly favours the hypothesis 

 of intrusion. Emphasis is placed on the sequence of rock-types 

 and on the chemical composition of the j)orphyries as confirmatory 

 of magmatic segregation and particularly of the theory of 

 differentiation in place. The ore-bodies are believed to be basic 

 segregations from the overlying quartz-porphyry intrusion, the upper 

 member of the composite laccolith, accumulated at its base by 

 gravity ; while the so-called " inclusions " found scattered 

 throughout its whole thickness are also regarded as units of 

 differentiation, and not xenoliths, as formerly believed. A detailed 

 study of the forms of these masses favours this conclusion. Those 

 masses scattered at higher levels are similar in composition to the 

 ore-bed at the base, and are supposed to have crystallized after 

 the magma had become too viscous to allow them to sink like the 

 earlier ones which formed the ore. An important point is here raised, 

 namely, that since no one disputes that single crystals, say of 

 magnetite, in a magma are formed by differentiation, that is by 

 aggregation of molecules from the solution, there is no reason why 

 comiDound masses of greater size should not have been formed in 

 an exactly analogous manner, and it is illogical to draw any 

 arbitrary line between such units of varying size by assuming that 

 the larger ones must be xenoliths. 



R. H. R. 



The Amisk-Athapapuskow Lake District. By E. L. Bruce. 



Canada, Department of Mines, Geological Survey Memoir 105. 



pp. iii -\- 91, with 7 plates, 4 figures, and a coloured map, 



Ottawa, 1918. 

 rpHIS area lies on the boundary of Manitoba and Saskatchewan, 

 -^ on the divide between three great rivers, the Churchill, 

 Nelson, and Saskatchewan. It forms part of the border of the 

 Canadian shield, about half being occupied by pre-Cambrian rocks 

 a,nd half by Ordovician dolomites. The ancient rocks are divided 



