STUDY OF IGNEOUS ROCKS. ' 275 



magmas during emption and solidification, the minerals produced in 

 chemicallv similar magmas ai-e not to be expedited to be always alike, 

 and the variations in composition are in this way understood. 



Having considered the possible chemical reactions that may give 

 rise to mineral compounds in rock magmas, the next step in the 

 treatment of the subject is a discussion of the process and results of 

 separation of various compounds or substances from magma solutions 

 upon change of physical conditions attending the ei-uption of magmas. 

 These may separate as gases, liquids, or solids, chiefly as solids. But 

 gases escape in large volumes upon the eruption of lavas, mostly as 

 water vapour. Thei-e ai'e other kinds in smaller, though often in 

 considerable amounts. The effects of this loss of gases .are: in the 

 cliemical composition of the rock magma, in the concentration of the 

 remaining substances, and in the viscosity of the magma, which may 

 increase notably upon loss of gas. 



Liquids, probably, do not separate as such from molten magmas 

 to any considerable extent. Apparently liquid silicates are miscible in 

 one another in all proportions, though suggestions that they may not 

 be have been advanced by some petrologists. It is known that liquid 

 sulphides and silicates are not miscible in all proportions at all 

 temperatures. And where sulphides exist in large amounts separation 

 in the liquid phase may take place with falling temperature. 



Separation of solids from solution depends upon the attainment of 

 a sufficient molecular concentration of substances to saturate the 

 solution. Saturation may be brought about in several ways : by 

 chemical reaction within the solution consequent upon a change of 

 chemical equilibriimi ; by change of temperature, usually by lowering 

 temjierature ; bj' cliange of pressure, either acting in an opposite 

 manner from temperature, or by affecting the gas content. 



Solids may separate when the point of saturation for them has 

 been reached, or the liquid may become supersaturated, and separa- 

 tion be delayed. In this condition separation is often induced by the 

 insertion of a solid of like composition, or of an isomorphous com- 

 pound, or by agitating. Such a condition of a liquid has been called 

 met ast able, and in this condition, as shoA\ai by Miers in laboratoiy 

 observations on liquids of organic con:ipounds, crystallisation of the 

 separating substance takes place, at relatively few points, and pro- 

 ceeds gradually, according to degree of concentration and other factors, 

 until comparatively large individuals are formed. If supersatiu'ation 

 proceeds without separation of solid phase, a point will be reached 

 when separation will take place spontaneously at many points in the 

 liquid and continue rapidly. This is the labile condition of the liquid. 

 When this condition is reached by a cooling, liquid crystallisation often 

 takes place suddenly as a shower of minute individuals, as observed 

 by Miers. The bearing of these facts on the textures of igneous rocks 

 is apparent, and a knowledge of the laws relating to the separation of 

 solids from liquids ; the order in which those of different substances 

 may follow one another in a mixed solution ; the separation of isomor- 

 phous compounds : and the shapes that may be assumetl by the 

 resulting crystals of various minerals lead to an understanding of the 

 textiu'e of igneous rocks. 



A supersaturated condition is more I'eadily obtained in more 

 viscous liquids, which are more apt to solidif}- without crystallisation. 



