206 AUSTRALASIAN ANTAECTIC EXPEDITION. 



grain size of 0-05mm., while localised areas of slightly coarser grains may be found. This 

 fine-grained product has been shown to be practically identical in kind with the much 

 coarser-grained plagioclase pyroxene gneiss at Madigan Nunatak, with average absolute 

 grain size of 0-30mm. The differences in the grain size (T5mm. and 0-22mm.) of the 

 amphibolites (No. 9 and No. 629), at Cape Denison, illustrate the same phenomena. 

 In these instances all the constituents of the rock have been uniformly enlarged. There 

 are also cases where certain constituents are enlarged at a relatively greater rate than 

 other constituents and heteroblastic rocks result. It is believed that the garnets in 

 the garnet amphibolite (No. 953) have formed by the aggregation of smaller crystals, 

 arid they are uniformly larger than the average crystal of hornblende in the same rock. 

 The large magnetite crystals in the amphibolites, Nos. 143 and 637, at Cape Denison, 

 are also enlarged to a greater degree than neighbouring minerals, and consequently 

 appear as porphyroblasts. An even more striking example is found in the localised 

 variation in the size of the felspar grains that have developed in the metamorphic contact 

 products, the biotite felspar gneisses, at Cape Denison. In three specimens from one 

 locality the average width of the felspar crystal may vary from 0-90mm. in one case, to 

 8mm. in the second, and 27mm. in the third. Similar examples are known in other areas 

 of crystalling schists and in glacier ice, and Grubenmann considers that increase in 

 crystal size corresponds with a striving towards a condition of smallest surface tension, 

 because the sum of free surface is decreased. 



A little evidence of solid diffusion through crystalline solids and artificial crystals 

 is given in Desch's report. But it is stated that experiments are lacking to prove the 

 occurrence of diffusion in minerals, even under favourable conditions, though indirect 

 evidence points to its possibility. The most favourable observations according to him 

 are those of schiller inclusions, as of magnetite in the olivine from peridotite, Isle of 

 Rum ; and in the hypersthene from norite, Labrador ; and also rutile in certain felspar 

 and pyroxenes. These examples are phenomena connected with igneous rocks. If 

 solid diffusion has been operative in rocks it is more likely to be traced in the rock 

 bodies that have been completely recrystaliised, without fusion, than in igneous rocks 

 which have merely suffered slight changes on cooling after solidification. We, there- 

 fore, proceed to summarise favourable instances from our study of metamorphic rocks. 



1 . The presence of large porphyroblasts which have grown during the metamorphism 

 and illustrated by 



(a) Large garnets, 5cm. broad, in the garnet gneisses at Stillwell Island and 



Garnet Point. 



(b) Large felspars (oligoclase-andesine), 27mm. long, at Cape Denison, in 



certain metamorphic contact products, the biotite felspar gneisses. 



(c) Large anorthite crystals, 2|cm. wide, which are set in a fine-grained augite 



amphibolite found on the moraines at Cape Denison. 



(d) Large magnetite crystals in the amphibolites Nos. 143 and 637, at Cape 



Denison. 



