Febbuaet 5, 1909] 



SCIENCE 



209 



cibles from which, hydrogen, or water 

 vapor, naturally escapes at high tempera- 

 tures. And muscovite is not formed pyro- 

 genetically in surface lavas, or, if so, to a 

 very small extent as compared with its 

 occurrence in rocks crystallized under con- 

 siderable pressure. From these facts it 

 must be concluded that the formation of 

 the acid orthosilicate (muscovite) in the 

 presence of polysilicates and free silica 

 must be assigned to the chemical activity 

 of hydrogen at high temperatures under 

 sufficient pressure to hold it in the liquid 

 magma solution. 



The same argument as to the action of 

 hydrogen in rock magmas applies to the 

 production of the other micas, biotites and 

 lepidolite. These compounds are complex 

 mixed salts, and the composition of biotite 

 involves the production of orthosilieates of 

 magnesium and iron, which are present 

 in biotite. These orthosilieates develop 

 in magmas together with metasilicates 

 of magnesium and iron— pyroxenes and 

 hornblende— and with uncombined silica, 

 quartz. The same compounds when alone 

 form olivine, which generally does not de- 

 velop in magmas with uncombined silica, 

 quartz, but probably occurs with quartz 

 oftener than has been supposed. In both 

 of these cases the production of orthosili- 

 cate of magnesium and iron in the presence 

 of "free" silica in magmas in which the 

 metasilicate might be expected to form is 

 probably due to the hydrolyzing action of 

 water at high temperature. That is, the 

 hydrogen at high temperature combined 

 with some of the silicon, that otherwise 

 would have united with magnesium and 

 iron as metasilicate, and formed orthosili- 

 cate of these metals and orthosilicate of 

 hydrogen. 



(Mg, Fe)2(Si03)2H-2H,0?± 



(Mg, lie),(Si04) +H4(SiOJ. 



Should conditions of saturation favor the 



separation of the magnesium-iron com- 

 pound in the solid phase, olivine would 

 crystallize; and with falling temperature 

 the hydrogen silicate would split up into 

 water (HjO) and silica (SiO,), with the 

 eventual crystallization of quartz. 



When the amount of silica is great as 

 compared with that of magnesium-iron, 

 orthosilicate it is possible for quartz to 

 separate before the orthosilicate, as is the 

 ease in many hollow spherulites and litho- 

 physEe, where fayalite, (Fe(Mg))2Si04, 

 is apparently almost the last mineral to 

 crystallize, and rests upon the surface of 

 abundant quartzes. The dependence of 

 these forms of crystallization upon the 

 presence of water in the magmas has been 

 clearly demonstrated. 



Other mineral compounds, whose pro- 

 duction in igneous rocks must be referred 

 to the hydrolyzing action of water, are 

 amphiboles, which, as Penfield has shown, 

 contain as an essential constituent notable 

 amounts of hydrogen. The development of 

 hornblende in igneous rocks appears to be 

 dependent on conditions similar to those 

 controlling the development of biotite, for 

 they commonly accompany one another in 

 rocks of intermediate composition when 

 either is present. The particular kind of 

 amphibole which forms in rock magmas 

 depends primarily on the proportions of 

 ele.ments present, and secondarily on at- 

 tendant conditions which produce varia- 

 tion in amphibole from one magma, which 

 is strikingly illustrated in the two igneous 

 rocks from Gran, Norway, described by 

 Brogger.^ The magmas have almost the 

 same chemical composition, yet one crys- 

 tallized into a mixture of hornblende and 

 lime-soda-feldspar, while the other crys- 

 tallized almost completely into hornblende, 

 which contains all the components of the 



= Brijgger, W. C, " Erupt. Gest. kp. Geb.," Vol. 

 III., 1899, p. 93, and Quart. Jour. Geol. Soc, 

 Vol. L., 1894, p. 19. 



