PETROLOGICAL NOTES ON FURTHER ROCK SPECIMENS. 315 



felspar or quartz. These reaction areas are usually found as inner zones next to the 

 zone of garnet. They show how the amphibole and the original basic felspar combine 

 to form the garnet and to give rise to a separation of quartz. That this separation of 

 quartz is an intermediate stage of the metamorphism and not the final one, is made 

 evident where garnet and the green hornblende abut without an accompanying acidic 

 reaction zone. The acidic material set free in the earlier reaction has been taken up 

 completely in the final one and hornblende and garnet are, in consequence, in intimate 

 juxtaposition. 



Exactly similar relationships are seen to hold wherever the biotite region has been 

 able to react with the original felspar. There are places where phalanges of biotite 

 mingle with clear siliceous matter (or highly siliceous felspar), and there are other places 

 where the biotite and garnet abut against one another. This is good evidence of the 

 complementary nature of the biotite and the hornblende ; the one being the more 

 ilmenitic derivative of the original pyroxene and the other the component richer in iron 

 and more especially magnesia. They show, too, how the original pyroxene has 

 progressively altered, first, setting free a swarm of ilmenite grains ; secondly, producing 

 a granulitic pyroxenic aggregate ; thirdly, the formation of green hornblende ; fourthly, 

 the subsidiary production of brown biotite by the concentration of ilmenite in the 

 ferromagnesian, and lastly the reaction between the new ferromagnesian and the original 

 basic felspar with the final production of a pink garnet but with the intermediate stage of 

 silica liberation and later phase of absorption of this matter. 



Some indication of the structure of the rock has been given by what has been 

 said above. But the general arrangement of the mineral masses is that these more or 

 less circular masses are set somewhat regularly in a felspathic base. In places two or 

 more ferromagnesian alteration zones have coalesced and then the same internal 

 structures are seen to be preserved but there is now no separation region of felspar. 

 The circular nature of the pyroxene and its derivative minerals sometimes gives place 

 to an elongated formation probably due to the effects of non-hydrostatic stresses 

 where the length and breadth of the masses are roughly in the ratio 2:1. Where the 

 pyroxene is not much altered the average grain-size is about 0-55 mm. This size is 

 preserved remarkably consistently throughout all the metamorphic stages, and finally 

 there has been produced a core of ilmenite, biotite, and hornblende about 0-5 mm. across 

 with a surrounding aureole of garnet from 0-05 mm. to 0-10 mm. wide. The distance 

 between the adjacent nuclei of these masses is about 0-8 mm., leaving an average space 

 of 0-10 mm. to 0-15 mm. occupied by the felspar. 



No. 980. A coarse Garnet-Plagioclase-Cordierite-Biotite Gneiss. 



A coarse-grained garnetiferous rock was collected from the summit of Stillwell 

 Island. The perfectly developed garnet crystals are set in a matrix of dense greyish 

 felspar. A contact region is present in the specimen examined, where a considerable 

 quantity of biotite comes into the rock and where the grain-size is much reduced. 



