La vis — On the Structure of Rocks. 147 



Somma may have noticed that most of the lavas along its crest 

 are coarse-grained, whereas most of those near the toe are fine- 

 grained. The reason, as at present with Vesuvius, is obvious. 

 From the almost impossibility of artificially producing the 

 felspars, Delesse, 1 Daubree, 2 and Sorby, 3 assert that they must be 

 the result of hydro thermal origin. Whether the actual presence 

 of water is necessary directly, or only as the means of increasing 

 the tension and pressure in the magma, seems at present un- 

 answerable. 



Anorthite was the most easily obtained, and corresponded in 

 characters exactly with the same mineral in lavas that have con- 

 solidated near the surface. This mineral, as is well known, is 

 rarely met with in true plutonic rocks. 



Quartz appears never to have been produced artificially, ex- 

 cept from solution in water of silicates of a glass at a high tem- 

 perature and pressure by Daubree ; and from the abundance of 

 fluid cavities seems to be the result of (in rocks) hydrothermic 

 origin under very great pressure. 



Leucite, although a mineral of local occurrence, is of deep 

 interest to the petrologist. It has never been met with amongst 

 furnace slags, except as a sublimation. M. Hautefeuille 4 obtained 

 measurable crystals by fusion of the components of leucite in 

 vanadate of potash. Fouque and M. Levy 5 obtained by igneous 

 fusion and rccuit without a flux. With the components of leu- 

 cite alone it was impossible to obtain the mineral, and this could 

 only be done by taking equivalent components of a mixture of 

 that mineral and pyroxene. This is a most important fact that 

 again helps to clear away the veil of mystery which overhangs 

 the genesis of many silicates. Most substances can be obtained 

 crystallized by one or more of four principal methods — from sub- 

 limation, by fusion, by evaporating a solution, and by cooling 

 down a solvent. The necessary temperature is highest for the 

 first, less for the second, and very much the lowest for the third 

 and fourth. Sulphur, to be obtained in crystals from fusion, 



1 Bull. Soc. Geol, 1857 and 1858, vol. xv., p. 728, 757, 769. 



2 Eapport sur les progres de la geologie experimentale, 1867, pp. 63 and 84. 



3 Brit. Assoc. Reports, 1880. 



4 Annales Scient. de l'Ecole norm. sup. 2nd'series, vol. ix., 1880. 



5 Comptes rendus, 1878. t. lxxxvii., p. 961. 



M2 



