J. S. Teall — Cheviot Quartz-Felsites and Augite- Granites. 119 



gressive crystallization is to increase the percentage of silica and 

 alkalies, to increase the potash relatively to the soda, and to diminish 

 the lime, iron, and magnesia in the liquid magma. Eocks of rhyolitic 

 composition may represent, so to speak, the mother-liquor out of 

 which the basic minerals have crystallized. 



Suppose, now, that the crystallization occurs under conditions 

 which admit of the mechanical separation by gravity of the crystals 

 and the magma in which they are formed. This would give rise to 

 some very interesting results.^ 



Consider the case of an internal reservoir of molten-rock, and for 

 the sake of simplicity, suppose the conditions of crystallization to be 

 realized in the upper portion. The basic minerals will form and 

 then fall by reason of their high specific gravity.'"^ On descending to 

 lower levels, they may be dissolved, and thus a variation in the com- 

 position of the originally homogeneous magma may be produced. 

 Diffusion will tend to restore homogeneity ; but the above cause, operat- 

 ing in the contrary direction, may be the more potent of the two, 

 and if so, a permanent difference in composition will be established. 

 When final consolidation occurs, a mass will be produced which will 

 vary in composition from top to bottom ; the most acid portion 

 occurring in the former, the most basic in the latter position. In the 

 Cheviot district the augite granites varj'^ considerably in composition 

 and different varieties sometimes appear to shade into each other. 

 The same feature may be observed in rocks which I cannot but 

 regard as igneous, in the Carrock Fell region in Cumberland. The 

 highly basic gabbros of Mosedale Crags appear to shade into the 



^ Separation may be effected without gravity. Suppose a half-consolidated 

 plutonic mass to become subject to the lateral thrust of which there is such striking 

 evidence in certain districts. The mother-liquor may be squeezed out of the rock as 

 water out of a sponge. In this way contemporaneous veins might be produced in the 

 plutonic rock, dykes in the surrounding rock, and lavas at the surface ^ 



^ The subject of the separation of crystals in a molten magma has been discussed by 

 many writers. It is important to notice that it must occur if there be any difference 

 between the specific gravity of the crystals and that of the magma in which they exist. 

 The only question that can arise is as to the geological signiticance of the operation. 

 Mr. Darwin deals with the subject in his Geological Observations (2nd edition, 

 p. 132), where he describes certain phenomena in the Galapagos Islands which he 

 attributes to this cause. He also quotes a statement from Von Buch that M. Dree 

 found, on melting lava, that the felspars always tended to fall to the bottom of the 

 crucible. The Pattison process for separating lead from silver is mentioned by 

 Mr. Darwin, and it certainly has a very interesting bearing on the subject under 

 consideration. 



Again, Mr. Clarence King (Systematic Geology, p. 678) says: "During an 

 eruption in the crater of Kilauea at the time of my visit, a fluid stream of basalt 

 overflowed fmrn the molten lake at the west end of the crater and poured eastward 

 along the level floor of the pit. Numerous little brauchlets spurted out from the 

 sides of the flow and ran along the depressions of the basaltic fluor, for a iew feet 

 and then congealed. I repeatedly broke these small branch streams and examined 

 their section. In every case the bottom of the flow was thickly crowded with 

 ti'icliuic felspars and augites, while the whole upper part of the stream was of nearly 

 pure isotropic and acid glass." The sinking of felspars in a basaltic magma strikes 

 one at first sight as an impossibility ; but the observations of Diu-win and King and 

 the experiments of M . Dree appear to prove the fact in a conclusive manner. We 

 must inter, therefore, that the specific gravity of the molten magma is less than that 

 of the felspars. 



