74 AUSTKALASIAN ANTAECT1C EXPEDITION. 



same proportion as in No. 630A. Epidote is often associated with the biotite, and 

 there is a subordinate amount of quartz; sphene, apatite, magnetite, and pyrite are 

 present. The felspar is nearly all perfectly clear, and in some cases two sets of lamellar 

 twinning are beautifully developed, especially in the porphyroblasts. The maximum 

 extinction angle that has been measured is 16, and the refraction is very close to, but 

 always less than, nitrobenzol (1-551). It is always above nelkenol (1-542). We, 

 therefore, consider it to be an oligoclase-andesine. The brown biotite shows very little 

 change to chlorite. 



In the amount of biotite and in the complete absence of hornblende this rock is 

 similar to the previous transition type, No. 10, and to the biotite zone described in the 

 junction specimen No. 372. There is much less quartz in this case than in No. 10, 

 while clear felspar replaces the cloudy felspar. There is more epidote than in the No. 

 372 example, but it is very likely that this rock has a similar origin and is a product 

 of metamorphic hybridisation. 



There can be no doubt that the clear character of the felspar is due to the meta- 

 morphism, and the size of the crystals has, therefore, no great significance. There is, 

 in fact, considerable variation in the size of the felspars from the same locality as No. 144. 

 In specimen No. 146-1 the average size of the felspar is about equivalent to the 

 porphyroblasts in No. 144 (Plate XII., figs. 1, 2, and 3), but the mineral composition 

 of the rock is similar to that of No. 144. In specimen No. 146-2 there is a still greater 

 development of the felspars ; but in this case there has not been a uniform development 

 and some crystals are much larger than others. In the hand specimen there is a 

 suggestion of a brecciated appearance, but in section there is no evidence at all of 

 crushing or cataclasis. No variation in the constituent minerals in the different 

 specimens has been noticed, while the specimens were collected within a yard or two 

 of one another. 



Primary relic felspar can usually be recognised, both in the granodiorite gneiss 

 and in the amphibolites, either by the mechanical alteration or by the saussuritisation. 

 On this ground alone we would have difficulty in maintaining an igneous origin for 

 the porphyroblasts. Further, as neither the amphibolite dykes nor the granodiorite 

 is porphyritic near the particular outcrop, it is impossible to apply Cole's explanation* 

 of the origin of porphyritic felspars in the biotitic schists, related to amphibolite, which 

 are associated with a porphyritic granite gneiss at Barna, County Galway. In Cole's 

 theory the felspar crystals were present in the original granite magma which threaded 

 and penetrated the surrounding schists, increasing their Si0 2 percentage. The felspar 

 phenocrysts, however, could not flow away and became stranded, one by one, in parallel 

 series in the schists. In the Barna granite the large felspars are orthoclase, while in 

 Cape Denison examples they are plagioclase ; but in both cases the matrix is chiefly 

 biotite, and the rock is related to amphibolite. In our case the origin of the large 

 felspars is connected with the origin of the biotite felspar rock. 



* " A Composite Gneiss near Barna (County Galway)," G. A. J. Cole, Q. J.G.S. LXXL, 1916, p. 183. 



