403 



mulate in the marginal portions, leaving the centre richer in those which 

 play the role of solvent medium. An example of this kind is furnished 

 by the Eskdale dyke (see ante, page 195), and it is worthy of note that 

 the difference in composition between the glass and the marginal portion 

 is exactly what is required by theory. 



SOKET'S principle may also be applicable to plutonic rocks. Homo- 

 geneous molten masses may become heterogeneous by diffusion of certain 

 constituents during cooling. Thus an intermediate magma may become 

 differentiated into a basic and an acid magma. 



An interesting question arises as to whether super-saturation or 

 over-cooling have taken place in igneous magmas which have subsequently 

 consolidated as crystalline rocks. LAGORIO is of opinion that the porphyritic 

 crystals in igneous rocks have grown rapidly in super-saturated solutions, 

 and that the granitic structure in plutonic rocks is the result of rapid 

 crystallization in over-cooled magmas under pressure. A prolonged dis- 

 cussion of these points in the present imperfect state of our knowledge 

 seems scarcely desirable in a work like the present. As regards the 

 latter point we may, however, remark that over-cooled silicate-solutions 

 under ordinary pressures are so viscous that anything like rapid crystal- 

 lization is impossible. Such solutions consolidate as glasses. 



Any account of the facts which throw light on the conditions under 

 which igneous rocks are formed would be incomplete without a description 

 of the remarkable synthetic experiments of Messrs. FOUQUE and LEVY. 

 The first attempts to reproduce crystalline rocks by pure igneous fusion 

 were made by JAMES HALI/D and GREGORY WATT< 2 > towards the close of 

 the last century and during the commencement of the present one. They 

 showed that if a basic igneous rock be melted and then slowly cooled it will 

 assume a more or less crystalline structure. HALL, moreover, established 

 the important fact that the fusing point of the artificial glass, formed by 

 melting an igneous rock, is lower than the fusing point of the original 

 crystalline rock or of the artificial crystalline product formed by the slow 

 cooling of the molten mass. The imperfect methods available for the 

 purpose of determining the precise character of the artificial products 

 prevented these early observers from deriving the full advantage from their 

 most important experiments. 



Previous to the experiments of Messrs. FOUQUE and LEVY it was very 

 generally believed that igneous rocks were not and could not be produced 

 by pure igneous fusion. This view had been insisted upon by many 

 observers, including DELESSE, DAUBREE and SoRBY.< 3 > It was held that 

 aqueous vapour played an important part in promoting the development 

 of crystals, even in volcanic rocks. The experiments referred to were 

 made with a gas furnace and a forced blast. By varying the arrangement 

 of the parts and the supply of air and gas a considerable range of 



(1) Trans. Roy. Soc. Edin., Vol. V. (1805). 



(2) Phil. Trans., (1801), p. 279. 



(3) Geol. Mag., Decade II., Vol. VII. (1880), p. 468, 



