STUDY OF IGNEOUS ROCKS. -73 



the compound in question, then its action in replacing part of the 

 potassium must be that known as hydrolysis, wliereby the hydrogen 

 ions from water replace metals in the salt through a process of double 

 decomposition. 



It is knowTi that the chemical activity of hydrogen even toward a 

 o-as, like oxygen, is greatly increased by rise of temperature ; hydrogen 

 being rather inert at ordinary temperatures. The relative activity of 

 hydrogen and potassium toward silicon is indicated by the fact that 

 the highest hydrogen silicate definitely known is the orthosilicate', 

 H^SiO^ (orthosilicic acid), whereas potassium commonly occurs in a 

 polysilicate KAlSisOg (orthoclase). It has been found impossible to 

 produce mica in open crucibles from which hydrogen, or water-vapour, 

 naturally escapes at high temperatures. And muscovite is not formed 

 pyrogen etically in surface lavas, or, if so, to a very small extent as 

 compared with its occurrence in rocks crystallised under considerable 

 pressure. From these facts it must be concluded that the fonnation 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, biotite and lepidolite. 

 These compounds are complex mixed salts, and the composition of 

 biotite involves the production of orthosilicates of magnesium and 

 iron, which are present in biotite. These orthosilicates 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 

 develop in magmas with uncombined silica, quartz, but probably occurs 

 with quartz oftener than has been supposed. In both of these cases 

 the production of orthosilicate of magnesium and iron in the presence 

 of '■ free silica " in magmas iit which the metasilicate might be ex- 

 pected to form is probably due to the hydrolysing action of water at 

 high temperature. That is, the hydrogen at high temperature com- 

 bined with some of the silicon, that otherwise would have united with 

 magnesium and iron as metasilicate, and formed orthosilicate of these 

 metals and orthosilicate ol hvdrogeu. 



(MgFe)3 (Si03)3 + 2H20"'(JVIg; Fe)2 (SiO^) + H^ (SiO^). Should con- 

 ditions of saturation favour the separation of the magnesium-iron 

 compound in the solid phase, olivine would crystallise; and Avith 

 falling temperature the hydrogen silicate would split up into water 

 (H3O) and silica (Si02), with the eventual crystallisation 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 case in many hollow spherulites and 

 lithophysae, where fayalite (Fe, (Mg))2 SiOj^ is apparently almost the 

 last mineral to crystallise, and rests upon the surface of abundant 

 quartzes. The dependence of these forms of crystallisation upon the 

 presence of water in the magmas has been clearly demonstrated. 



Other mineral compounds whose production in igneous rocks must 

 be referred to the hydrolysing 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 



