126 MR. C. E. TILLEY ON THE GEAlsriTE-GNEISSES [vol. Ixxvii, 



therefore, that these bands of the Lincoln area may rej^resent 

 dolerites of anterior origin which have become engulfed and en- 

 tangled in the acid gneisses, suffering at the same time an intense 

 thermal metamorj^hism, must in the present state of knowledge not 

 be excluded. 



On the other hand, rocks bearing a remarkable resemblance to 

 those which are under discussion have been quite recently described 

 in detail by Dr. F. L. Still well from Adelie Land ( Antarctica ).i 

 This author is able to produce indisputable evidence that the meta- 

 morphosed dolerites are a series of dyke-rocks in that thej definitely 

 ramify throughout the enclosing rock, and further that the enclos- 

 ing rock itself is a metamorf»hosed sediment. Thus at Cape Gray, 

 these dykes invade a rock which is now a garnet-cordierite gneiss^ 

 of the sedimentar}^ origin of which there can be little doubt. The 

 metamorphism of the whole series is ascribed to the conditions pre- 

 vailing in Grubenmann's ' kata-zone.' 



As against a dyke origin for the Lincoln rocks, it can be defi- 

 nitely stated that the enclosing rocks (the acid gneisses) owe their 

 gneissic foliation to the stages of consolidation, and are essentially 

 primary gneisses. There are no notable features in these rocks 

 that can be assigned to any high-grade metamor^^hism. To what- 

 ever origin these dolerites can be ultimateh^ ascribed, the essential 

 conditions for their metamorphic features have been high tempe- 

 ratures and an uniform pressure, to the exclusion of any important 

 shearing-stress. 



The balance of evidence, and, for a posterior dyke-origin, the 

 necessity of adducing a period of high-grade metamorphism, sub- 

 sequent to the consolidation of the igneous gneisses, for which 

 there is no other evidence, are in favour of the view that these 

 dolerites (like the amphibolites with which they are associated) are 

 inclusions derived from a pre-existing rock-mass entangled in, and 

 thermally metamorphosed by, succeeding acid gneisses. 



The close similarity of the metamorphosed Lincoln dolerites 

 with the Adelie Land series may here be further remarked. The 

 same ' relict ' dusty pyroxenes are again observed, as also the 

 granulitic secondary pyroxene. It is of further interest to note 

 that Dr. Stillwell also records hj^persthene as a dissociation -product 

 of the primar}^ augite. As he remarks, the resultant secondary 

 pyroxene is thus more aluminous. The production of garnet is 

 also observed ; but the formation of this is ascribed to the felspar- 

 pyroxene interaction. From the descriptions it would, however, 

 appear that quartz is not always present as an associate of the- 

 garnet. If, as in the Lincoln rocks, garnet has been produced hj 

 an intramolecular change of the primary pyroxene, the resultant 

 secondary augite is of necessit}^ nearer diopside in com^position, 

 and this is probably the case even when hypersthene is a further 

 product of this augite transformation. 



^ ' The Metamorphic Eocks of Adelie Land ' Sci. Eep. Austr. Antarctic. 

 Exped. ser. A, vol. iii, pt. 1, sect. 1 (1918) p. 169. 



