The Gneissose- Granite of the Himalayas. 351 



In order to realize the full significance of the presence of un- 

 foliated dykes in the foliated granite, it is necessary to bear in mind 

 that the dykes appear to have been derived from the same source 

 of supply as the granite itself. Mr. Teall tells us at p. 45 : — " It 

 follows from the above description, that there is very little differ- 

 ence in composition between the dykes of this type and the granitic 



rocks of the Criffel and Dalbeattie mass It is probable, 



therefore, that the structural differences between the porphyrites 

 and the hornblende granites or quartz diorites are due to differences 

 in the conditions under which the magmas consolidated. If we 

 suppose that an intrusion of the granitic magma took place in dyke 

 form after the greater part of the hornblende, biotite, and plagioclase 

 had separated out, we shall have a satisfactory explanation of the 

 difference in structure between the porphyrites and the granitic 

 rock. In applying this theory, howevei', it must be remembered 

 that dykes of the type now under consideration occur not only in 

 the sedimentary rocks, but also in the granitic masses. Moreover, 

 dykes are found in the foliated area unaffected by the foliation. 

 Such dykes cannot be regarded as in any way representing apophyses 

 of the granite. They belong to a later phase of igneous activity. 

 The source of supply was probably the same as that from which 

 the main igneous mass was derived ; but if so, it must have been 

 tapped at a time when crystal building had proceeded to a con- 

 siderable extent." 



It would appear, therefore, that although the foliation of the 

 granite was affected after the formation of the acid veins, it was 

 completed before the porphyrite dykes were intruded into the 

 granite ; and as the source of supply of the dykes and of the granite 

 was probably the same, no great lapse, measured by geological 

 time, is likely to have intervened between the foliation of the 

 granite and the intrusion of the unfoliated porphyrite. 



Mr. Teall, at p. 43, remarks : — " The interstitial movement which 

 has resulted in the production of foliation, and which is proved by 

 the field evidence to have been produced by earth-stresses operating 

 after the rock had reached its present position, and after the forma- 

 tion of the acid veins, has not affected all the constituents alike. 

 The quartz and alkali- felspar which, as we have already seen, were the 

 last constituents to solidify, are those which have yielded most to the 

 deforming stresses. They show signs of crushing, and frequently 

 pass into thin micro-crystalline or even crypto-crystalline streaks, 

 which wind round the comparatively uncrushed crystals of oligo- 

 clase. The distribution of the ferro-magnesian constituents has 

 been modified by the movement. Instead of being irregularly 

 distributed through the rock they tend to form flaser in association 

 with the micro-crystalline aggregate of quartz and alkali-felspar. 

 The oligoclase has been little, if at all, affected by the movement. 

 These facts clearly prove that the minerals which yield most readily 

 to the deforming forces are those which consolidate last. It is 

 probable that the pressure acted before the roclc-mass had actually 

 cooled." The italics are mine. 



