Lavis — On the Structure of Rocks. 149 



B, but as this separates, A can no longer remain in solution, so 

 this also separates until the magma is deprived of as much of the 

 elements as these minerals A + B can take up, and the glass is 

 then suitable for the growth of which comes next, and in its turn 

 may be followed by I), and so on. The resulting rock will be 

 composed of the minerals A + B + C + D, &c. Let us again start 

 with the same magma, and suppose that condition 2 comes into 

 play, which is favourable to the formation of A, which will separate, 

 exhausting the magma to a point that it is suitable to the forma- 

 tion of X, in preference to any other, which now carries the 

 exhaustion on, till the magma approaches Y in composition, which 

 in turn continues the exhaustion, till the unformed material is 

 suitable for the crystallization of D. We should thus obtain a 

 rock containing the minerals A + X + Y + D, both of which would 

 be identical in ultimate chemical composition. Now, condition 1 

 may have been favourable to rapid expansion, and eruption such 

 as pumice results from, whilst condition 2 we may take to represent 

 the gentle outflow of lava. The reality of this somewhat rough 

 illustration will be more apparent if we compare the vitreous 

 pumices of Phases III. and VI. of Monte Somma, in which leucite 

 is absent, and sanidine abundant, with the highly leucitic basalt 

 lavas of the same volcano, in which sanidine at the most is a very 

 unimportant element, remembering at the same time the practi- 

 cally complete identity in chemical composition of the mass of 

 either. An interesting point in connexion with this is the fact that 

 Messrs. Fouque and M. Levy obtained a leucitic rock from fusing 

 together orthoclase and biotite. Prof. Samuel Haughton 1 was, I 

 believe, the first to treat the mineralogical composition of a lava 

 on the principle of the exhaustion of the element of the magma or 

 paste, the different minerals competing for certain oxides which 

 are necessary for their formation, so entirely devoting himself, with 

 remarkable ingenuity to the chemical side of the question, but 

 disregarding the physical, which, however, hardly entered into the 

 scope of the subject discussed. We must, however, not forget the 

 varying conditions under which cooling, in an igneous rock, takes 

 place, such as time, pressure, water, volatile acids, and their corre- 

 sponding salts, which must be most important elements in modify- 



1 Op. cit. pp. 68 and 138. 



