401 



It appears from these analyses that there is no very marked 

 difference between the composition of the rock and of the glass in 

 the basic rocks. In every case, however, we notice a slight tendency 

 of the K 2 0-silicate to increase in the glass. The marked tendency 

 of the potassium silicate to remain behind in the mother liquor lends 

 support to LAGORIO'S opinion that such a silicate has a greater claim 

 to be regarded as the solvent medium than any other compound. 

 It is important, however, to note that the mother liquor of inter- 

 mediate and acid rocks contains more silica than that required to 

 form orthoclase. Neglecting the other constituents which may be 

 regarded as having crystallized out, the ultimate residuum corresponds 

 to a mixture of quartz and orthoclase. Let us now assume that a 

 fused mixture of quartz and orthoclase will, on cooling, behave as 

 a mixture of fused nitre and chromate of potassium. If the two 

 substances are present in eutectic proportions they will consolidate 

 simultaneously and, conversely, if they consolidate simultaneously they 

 must be present in eutectic proportions. If they are not present in 

 eutectic proportions the compound in excess will first crystallize out 

 and then the eutectic mixture will consolidate. 



Now in the body of this work we have shown that micro-pegmatite 

 plays the role of ground-mass in many dioritic rocks, as for example in the 

 so-called " syenites " of Charnwood Forest, in the augite-diorite of Carrock 

 Fell ; and sometimes also in the enstatite-diorite of Penmaenmawr (see 

 Fig. 2, Plate XXXV.) Moreover, it plays a similar role in the granophyres 

 of Ennerdale, Carrock, and the Western Isles of Scotland. Again, in the 

 enstatite-diorite of Penmaenmawr micro-pegmatite forms segregation veins 

 and nodules. Mr. COLE has described a similar occurrence of pegmatite 

 in " Stanner Rock." W The matrix of many felsites which do not show 

 a micro-pegmatitic structure agree in chemical composition with a mixture 

 of quartz and felspar. If we accept the point of view here suggested we 

 have a very satisfactory explanation of the fact that in the acid rocks the 

 order of consolidation of quartz and felspar is not constant. When quartz 

 is present in excess of the eutectic proportion it will be the first mineral 

 to crystallize out ; when orthoclase is present in excess it will be first 

 formed. The hypothesis suggested above brings so many facts into 

 relation with each other that, notwithstanding its inability to explain all 

 the facts, the present writer is disposed to attach some importance to it. 

 If pegmatite (HAUY), whenever it occurs in an igneous rock, is an eutectic 

 mixture in GUTHRIJS'S sense, then it should have a constant composition. 



To determine the composition of pegmatite the present writer has 

 analysed a portion of a specimen, given to him by Mr. COLE, from a 

 vein in the diabase of Stanner rock, near Kington. Microscopic exam- 

 ination shows that the specimen consists of quartz and felspar intergrown 

 so as to form a very fine variety of graphic granite, verging on micro- 



(1) G.M,, Decade III., Vol. III. (1886), p. 220, 



