THE METAMORPHIC ROCKS OF ADELIE LAND ST1LLWELL. 79 



of felspar in the two portions of the rock is precisely the same. The felspar zone is 

 marked more by the absence of the biotite and hornblende rather than by the felspar 

 itself. Small crystals of sphene and apatite are present in the felspar zone. Apatite 

 crystals and small pieces of felspar are also included in the magnetite. With the gradual 

 increase in biotite and hornblende the felspar zone passes out into the normal biotite 

 amphibolite. 



In this example the rock is obviously part of the metamorphosed dyke series. It 

 does not seem possible to account for the zonal structure on any primary igneous 

 hypothesis. The magnetite crystals with a sphene rim are definite metamorphic 

 products, and the clear felspar is also a product of the recrystallisation. There is, 

 therefore, no reason to suppose that an association of these two products is anything 

 else than a metamorphic structure. The formation of this structure in these circum- 

 stances involves a migration of certain material. It is, in fact, a small differentiation- 

 magnetite centres have been enriched in magnetite and the biotite and the hornblende 

 have been repelled from the felspar zone. The process of metamorphic differentiation 

 in this case has involved the force of crystallisation. 



The magnetite nucleus of sphene crystals is a normal feature in most examples of 

 the amphibolite series at Cape Denison. The Ti0 2 content of the primary ilmenite 

 has combined with the felspar, producing sphene and hornblende, and it is, therefore, 

 readily understood why the sphene surrounds the nucleus of magnetite or relic ilmenite. 

 The relatively large crystals of magnetite with only a thin and often incomplete rim 

 of sphene are abnormal in amphibolites Nos. 143, 637, and abnormal conditions must 

 be pictured during their formation. It is certain that the Ti0 2 content of these examples 

 is not less than in the normal amphibolites, because they possess a high sphene content. 

 The abnormal conditions have permitted certain magnetite crystals to enlarge them- 

 selves by attracting smaller magnetite crystals, and diffusion of magnetite, which is 

 prevented in the normal case by a sphene shell, has occurred. We, therefore, suppose 

 that the rate of diffusion of the magnetite molecules in these abnormal cases has been 

 more rapid than the rate of reaction which produces the sphene. When the supply 

 of magnetite molecules around any one centre has been nearly exhausted the sphene 

 rim has become attached to the large crystal. 



i 



13. REVIEW AND DISCUSSION OF FIELD CHARACTERS. 



It is desirable to review the field characters in the light of the dyke origin of the 

 bands. This metamorphosed series of dykes differs from a normal parallel system 

 in the frequency and magnitude of the breaks in the surface outcrops. A normal dyke 

 channel may here and there swell out into local bulges, but the general appearance of 

 the bulges at Cape Denison, and the sharp, irregular way in which the bulge may 

 terminate, seem to indicate that the bulges are not normal dyke swellings. In following 

 the trend of the dyke we find no dyke in many places where we expect dyke, and, in 



