ANNITERSARY ADDRESS OF THE PRESIDENT. 79 



all cases this group of minerals is the first to solidify ; even in the 

 thin sahlbands of tachylyte we note the cloudy agglomerations of dark 

 dust, globulites or trichites, which indicate incipient differentiation. 

 These often, when well marked, are surrounded by lighter zones, 

 indicating that the segregatory process has continued after motion 

 was arrested in the mass ; but we may remark that, in the perido- 

 tite group, the presence of a large amount of magnesia appears to 

 have been unfavourable to the complete separation of the iron- 

 oxide, so that a large quantity has remained as an iron-silicate in 

 such minerals as olivine, enstatite, &c. There is usually as much, 

 sometimes more, iron in a peridotite than in a basalt ; yet a slide of 

 the latter exhibits many more granules of iron-oxide than the former. 

 Olivine appears to consolidate at a high temperature ; but in the 

 rocks rich in magnesia the bisilicates of the enstatite group, and 

 perhaps those of the pyroxenic, appear commonly to have crystal- 

 lized before it, though the difference cannot have been very great, 

 since these minerals occasionally include (as in the well-known 

 bastite-rock of the Harz) granules of olivine. If, however, the con- 

 stituents of felspar are present in any appreciable quantity, then the 

 olivine is anterior in solidification to the above maguesian bisilicates ; 

 for in the picrite group they frequently include grains of it, as does 

 a brownish mica which occurs occasionally. As a rule, the felspars, 

 including nepheline and leucite, when their constituents are present 

 in large quantities, appear to separate out at an early period ; they 

 are then generally anterior to the pyroxenic mineral, and, what is 

 remarkable, the more basic (and in the case of the true felspars the 

 more fusible varieties) separate out before the more acid, so that 

 the remaining magma contains a higher percentage of silica than 

 the separated minerals. In accordance with the same principle and 

 as an extreme case, quartz usually solidifies last in order. AYe find, 

 however, even in rocks of tolerably uniform structure, whether 

 coarse or fine, not unfrequent anomalies, so that it is almost impos- 

 sible to draw up a table of minerals in the order of their solidifi- 

 cation ; and when we study those which occur porphyritically, the 

 dif&culties become greater. The following table exhibits some of 

 these anomalies : — 



Minerals occurring 

 Composition of Ground-mass *. porphyritically *. 



^ ., ,., r\f • f Enstatite, auffite, hornblende, 



Fendotite ... Ohvme S biotite. 



^ , ., X 1- J -i. 4. ■ -L f Olivine, enstatite, aueite, la- 



Jjolertte Labradontet+augite | ^radorite. 



^y^nite Orthoclase-t-pyroxenic mineralj g^^^^ ^.^^^^^^ 



(also biotite) J 



^. ., _,! • 1 I • • 1 f Same minerals, but often the 



^^^^^^^ Plagjoelase + pyroxenic mmeral piagioclase is a more basic 



(also biotite) ( ^.^1 ^^^ byperstbene. 



{Same minerals, hut quartz 

 only in bemicrystalline or 

 glassy yarieties. 

 Tonalife Minerals of diorite + quartz Id. 



* Oxides of irorr and spinel group omitted. 



t Name used generically ; may include anortbite. 



