312 



in a plutonic mass would not be the result of the imperfect mixing of 

 the original materials ; but would be clue to differentiation taking place 

 in a magma which may have been perfectly uniform in composition. 

 In connection with this subject we may also refer to the development 

 of foliation in plutonic rocks by differential movement accompanying 

 intrusion. The foliation may consist merely in a parallel arrangement 

 of certain constituents of the granite, as for example the micas and the 

 tabular felspar crystals ; or it may consist of a parallel banding. In the 

 latter case it is of course necessary that the magma should not have been 

 homogeneous at the time of intrusion, as the bands vary in mineralogical 

 composition. An admirable illustration of the phenomena referred to 

 may be seen in the Langesundsfjord (Norway) <*> near the margin of a 

 mass of undoubtedly intrusive augite-syenite. Bands of a lighter and 

 darker colour alternate with each other so that the parallel structure is 

 often as perfect as in many crystalline schists. 



Granites may be either porphyritic or non-porphyritic. The porpby- 

 ritic constituent is orthoclase. It occurs as a rule in more or less tabular 

 crystals in consequence of the conspicuous development of the clinopinacoid. 

 This is the case in the well-known Shap granite and in many of the Cornish 

 granites. In the building of granite zircon and apatite belong undoubtedly 

 to the earliest period of crystal development. They occur as inclusions 

 in the other constituents. Sphene also when present belongs as a rule to 

 the same period. The ferro-magnesian minerals follow next and last of 

 all the felspar and qunrtz. The relations of the felspar and quartz are 

 somewhat complicated and no definite rule can be laid down to express 

 their order of formation. Plagioclase in almost if not in all cases precedes 

 quartz and generally, but not invariably, precedes orthoclase. In the 

 porphyritic granites orthoclase belongs to two periods. The porphyritic 

 orthoclase is idiomorphic; the orthoclase of the ground- mass is generally 

 allotriomorphic. It is commonly stated 'that the quartz of granites forms 

 the matrix in which the other constituents lie embedded ; that it was the 

 last mineral to form and that it accordingly takes its shape from the 

 disposition of adjacent crystals. This is by no means so universally true 

 as is commonly supposed. We frequently find that the quartz and the 

 orthoclase have mutually interfered with each other so that neither 

 mineral shows good form ; sometimes they have intercrystallized so as to 

 form micro-pegmatite as in some of the granites of Mull (granophyres) 

 and the Mourne Mountains. In the granites with two micas (muscovite- 

 biotite granites) the quartz occasionally shows more or less definite form 

 and is idiomorphic with respect to the orthoclase. It thus appears that 

 in some cases the quartz has succeeded the orthoclase ; in other cases it 

 has preceded the orthoclase ; in others the two minerals have crystallized 

 simultaneously. Idiomorphic quartz is rarely if ever found in the biotite- 

 and hornblende-granites. Microcline bears very much the same relation 

 as orthoclase to the other constituents of the rock. Tourmaline, topaz and 



(1) BROGGER, Die silurischen Etagen 2 und 3, p. 327, 



