REVIEWS 719 



than any of the others, it is nearer the Australian (less than i per cent.), the Paris 

 basin (1.70 per cent.) and the French Creek, Pa. (2.23 per cent.) glauconites than they 

 are to the Grodno, Russia, material (7.57 per cent.). Recalling Murray and Renard's 

 observation that the potash content of recent glauconites depends largely on the com- 

 position of the coastal rocks, he shows that the surface, at the time of deposition of 

 the Mesabi iron-bearing formation, consisted of rocks very low in potash. His final 

 conclusion, therefore, is that the Mesabi mineral is entitled to the name glauconite. 

 Spurr, J. E. "A Consideration of Igneous Rocks and Their Segregation or 



Differentiation as Related to the Occurrence of Ores." Trans. Am. Inst. 



Min. Eng., Vol. XXXII , advance separate, 53 pp. 



The author discusses briefly the general relation of ore deposits to igneous rocks, 

 pointing out that the ores were all originally derived from igneous rocks, and that 

 most existing ore deposits are closely associated with areas or belts of such rocks. 

 The sedimentary rocks, as a rule, contain a lower percentage of the metals than do 

 rocks of igneous origin. 



The differentiation of igneous rocks is then discussed, and the order in which the 

 various minerals crystallize out of a molten magma is noted. The concentration of 

 commercially valuable minerals by segregation within molten masses previous to their 

 consolidation, is discussed in some detail, the metals being separately treated. The 

 preference of concentrations of iron, chromium, nickel, copper, platinum (and proba- 

 bly cobalt) for the more basic igneous rocks, and that of molybdenum, tin, and tungsten 

 for the acid rocks, is stated. 



The author then takes up an original contribution to the study of ore deposits — 

 the general preference of gold for the more siliceous igneous rocks, and the relation- 

 ship of certain gold quartz veins to rocks of undoubted igneous origin. Studies in 

 certain Alaskan and other districts have convinced him that many gold-quartz veins 

 are to be regarded simply as extremely acid igneous rocks — the result of magmatic 

 segregation — and correlative to the extremely basic rocks (pyroxenites, etc.) at the 

 other end of the series. This theory is discussed in detail, and supporting observa- 

 tions, drawn from various districts, are given. This portion of the paper is sum- 

 marized by the author 'in the statement that " although gold is present in all 

 igneous rocks, and may be unequally distributed in any of them, yet the conditions 

 for concentration by magmatic segregation become more favorable in proportion as 

 the rock becomes more siliceous, and become most favorable in what has been 

 shown to be the extreme siliceous product of rock-differentiation — in quartz-veins 

 or dykes." 



The merging of deposits formed by magmatic segregation into certain types of 

 contact-deposits, and into deposits of gaseo-aqueous origin, is noted. The sequence 

 of rock types in the volcanic eruptions of an area is then discussed, leading up to the 

 relations between this sequence and the sequence of metalliferous deposits in the 

 same area. The conclusion is then drawn that "by magmatic segregation the metals 

 of commercial value, as well as the commoner rock-forming elements, are irregularly 

 and to a certain extent independently concentrated in certain portions of the earth's 

 crust. Such portions, characterized by the relative abundance of certain metals, may 

 be called metalliferous provinces." These "metalliferous provinces" may be more or 

 less closely identified with the different " petrographic provinces." 



