300 THE EVOLUTION" OF PETROLOCilCAL IDEAS. 



negative it, as tlie conditions of the experiment are cei'taiiily very 

 different from those under which the rocks are produced. 



Quartz and ortlioclase have not as yet l)een formed by pure igneous 

 fusion. IMie melting point at atmospheric pressure of a mixture of 

 quartz and orthoclase is a])Ove tliat of ])asalt, and yet we know from 

 the occurrence of anguUu" fragments of l)asalt in granophyre that the 

 consolidating point of the mixture under certain conditions of pressure 

 is below that of the fusing point of basalt under the same conditions. 

 If, as Professor La^winson-Lessing's calculations suggest, the forma- 

 tion of felspathic minerals is accompanied by an increase in volume, 

 and the formation of ferro-magnesian minerals by a decrease in volume, 

 pi'cssure will lower the fusing point of the former and raise that of 

 the latter, so that under plutonic conditions the relative order of con- 

 solidation of acid and basic magmas may be the reverse of that under 

 volcanic conditions. Magmas usually contain water, and somc^times 

 other volatile constituents (such as chlorine, boron, fluorine, etc.), 

 whose importance in determining the fluidity and the molecular group- 

 ing of the constituents has been generally rec'ognized since the pul)li- 

 cation of the classic paper "'' Sur les Emanations Volcaniques et Metal- 

 liferes," by Elie de Beaumont. When separated from the magma 

 these constituents exercise most important metamorphosing and min- 

 eralizing effects, as is well seen in the phenomena accompanying the 

 formation of tinstone and apatite veins, in the development of zeolites, 

 and in the production of large masses of kaolin. But so long as they 

 remain in the magma they must be regarded as belonging to it and 

 playing their part along with the other constituents in producing the 

 final result. 



The application of the theory of solutions to igneous rocks is com- 

 l^licated in many ways. We are ignorant of the manner in which the 

 constituents revealed Iw analysis are distributed in the molten magmas, 

 and of th(^ changes which take place in the molecular groupings as the 

 temperature approaches the point of saturation. M. Le Chatelier has 

 recently suggested that granite furnishes an illustration of the phase 

 rule, and may be regarded as a stable system of three phases ((juartz, 

 feldspar, and mica), made up of the three components — silica, alumina, 

 an^ potash. Few petrographers will admit that the case can be put 

 as simply as this. No d()u])t the consolidation of igneous magmas is 

 governed by the phase rule, but in the majority of cases the number 

 of components, on any view as to their nature, is too great to make 

 the rule of much practical value. Another cause of complication arises 

 from the fact that the physical conditions have often changed during 

 the process of consolidation, thus giving rise to the phenomena of 

 resorption; and yet another from the absence of assurance that the 

 minerals seen in a rock have in all cases been developed fi-om a magma 

 having the composition represented by the hulk analysis. 



