REVIEWS 3 i i 



of hornblende, and also many rock structures of characteristic habit. 

 These structures are proved to be the result of external conditions of 

 crystallization and also of chemical composition, both in qualitative 

 and quantitative sense. The order of crystallization of the individual 

 minerals depends on no one factor, such as "fusibility" or "acidity," 

 but is the result of a complex equation in which, perhaps, the most 

 important element is the ratio of the quantities of the several com- 

 pounds dissolved in and composing the solution. One and the same 

 compound can begin to crystallize out sooner or later than another 

 according to the amount which is present. The order of crystalliza- 

 tion is different in different magmas, and different substances have dif- 

 ferent capacity for forming saturated solutions in an aluminosilicate 

 magma. In certain cases temperature has an important influence : 

 magnetite, for instance, forms a saturated solution best at temperatures 

 below iooo° C. At higher temperatures it crystallizes out after 

 olivine. Amorthite crystallizes out more easily at a higher tempera- 

 ture (over iooo°). The process is obviously much complicated by the 

 changes in the magma itself as a solvent, by progressive crystallization 

 of the compounds composing it. 



The following are a few of the principles defined by observations 

 up to this date ; but final laws of silicate saturation can only be attained 

 by many experiments of character similar to these, which, as in the 

 advanced synthetic work of organic chemistry, shall have thrown light 

 on the structural formulae and atomic relationships of the silicates. 



i. Corundum, spinel, sillimanite, and cordierite in magmas super- 

 saturated with alumina, are the first products of crystallization. Spinel 

 and sillimanite crystallize before corundum. 



2. Magnetite at a temperature below iooo° crystallizes out in pro- 

 portion to the supersaturation of a solution with ferric iron and to the 

 amount of other iron compounds. It crystallizes sometimes before and 

 sometimes after augite and plagioclase according to their relative 

 amounts. 



3. The different orthosilicates of the type Me 2 Si0 4 (olivine, etc.) 

 crystallize first from a magma not supersaturated with alumina. 



4. Rhombic pyroxene develops earlier than augite, if the molecular 

 ratio of magnesia (and ferrous iron) to lime is about three or more. 



5. The crystallization of augite is very variable. 



6. In a magma with haiiyne {^ per cent.) in excess of anorthite 

 (23 per cent.) the haiiyne crystallizes first. 



