Frederick Guthrie on Eutexia. 481 



the silica, or an acid saturating lime or magnesia, or partly in 

 the one predicament and partly in the other. As "Wollastonite 

 occurs along with amphibole and augite, there does not appear 

 sufficient ground for this essential difference on the replace- 

 ment theory. 



I submit that, according to analogy, we should regard com- 

 pound rocks and minerals, other than sedimentary rocks, as 

 representing various kinds of eutectic alloys. We may for 

 the sake of argument start from some known definite rock 

 which is a true chemical compound, such as orthoclase, and 

 trace the probable reactions between this and other rocks ; or 

 we may take a rock, like granite or syenite, of indefinite compo- 

 sition, and examine the probable assortment of its constituents 

 on cooling. Starting with the melted mass, my experiments 

 w T ith salts have, I maintain, established that the clear molten 

 granite will in cooling throw off as solids atomically definite 

 salts, the last alloy to remain liquid being the eutectic alloy, 

 which is constant only in composition in the sense in which 

 such a body is so. It would follow therefore that the micas 

 are the eutectic alloys of the proximate elements, or salts of 

 the melted granite. Quartz and felspar being both molecu- 

 larlv constituted, undergo solidification before mica. On 

 account of the enormous effect which even a little water has 

 in reducing the melting-point * and because silica is soluble 

 in water, it is probable that the order of solidification is first 

 felspar, and then quartz, and lastly mica. Quartz is thus 

 speaking en gros pseudomorphic to felspar, and mica to both ; 

 all three are sometimes in this mass-sense pseudomorphic to 

 slate, and mica is especially so. If fused nitre be saturated 

 with nitrate of lead and sulphate of potash, and gradually 

 cooled, sulphate of lead first separates, then sulphate of potash 

 with sulphate of lead, then the eutectic alloy of all three. 

 The sulphate of lead represents the felspar, the sulphate of 

 potash with sulphate of lead represents the quartz, and the 

 eutectic alloy represents the mica (see also § 38, Phys. Soc. 

 1874). Tourmaline and hornblende are probably both eutectic, 

 whilst olivine is probably not. 



§ 231. As to the great rock-masses themselves, if we assume 

 that the earth is a drop of fused rock alloy with a solid crust, 

 and that its history had been one of gradual undisturbed 

 and symmetrical cooling, it is clear that, apart from sedimen- 

 tary formations, the oldest solid rocks are those formed nearest 

 the surface. But the disturbing causes are manifold: the earth- 

 crust is under stress on account of the loss of volume on 

 cooling. This strain is unequally distributed, on account of 

 "paubree, Geologie experimentale . Also my next memoir, §§ 339-345. 



