208 AUSTRALASIAN ANTARCTIC EXPEDITION. 



6. The highly localised nature of the reactions which result in the formation of 

 garnet, etc., in the garnet hypersthene felspar gneisses at the Cape Pigeon Rocks and 

 Stillwell Island, indicate that the reactions have occurred in solid rock and the molecular 

 supply is only provided from a very limited range. 



7. The manner of change in the isomorphous mixture of plagioclase, from labradorite 

 to andesine or oligoclase, during the reaction of labradorite with augite in the production 

 of garnet in the garnet plagioclase pyroxene gneiss (No. 935) from Stillwell Island, 

 indicates diffusion in the solid state. 



8. The development of felspar intergrowths in metamorphic rocks, similar in 

 character to some of the micrographic intergrowths, probably indicates solid diffusion. 

 The intergrowths are especially abundant in the garnet hypersthene felspar gneisses 

 at Cape Pigeon Rocks and Stillwell Island. These intergrowths bear some analogy to 

 certain pearlitic intergrowths which have developed with conditions permitting solid 

 diffusion. Such pearlitic intergrowths of iron carbide in phosphoferrite (solid solution 

 of iron phosphide in iron) were produced by Stead* during the cementation of an ingot 

 of iron containing 2 per cent, phosphorus and a little carbon. During this process a 

 small iron core was not penetrated by the external carbon, and it gave up even the small 

 percentage of carbon that it already had, and fan-shaped pearlitic intergrowths were 

 produced along the junction of the altered and unaltered portions. 



Though we can thus secure reason to suppose that solid diffusion can occur in 

 crystalline minerals, we cannot find in solid diffusion the whole cause of the phenomena 

 mentioned. In some of these cases the metamorphic diffusion products are highly 

 schistose. This means that solid diffusion has not merely resulted in indiscriminate 

 mixing of molecules, in the manner tending to reduce heterogeneous systems to homo- 

 geneous rocks. The operation of microscopic solution and deposition under Riecke's 

 principle, already mentioned, helps us to picture the development of the schistosity ; 

 but there is something else involved in the production of some of the structures. 



3. Force of Crystallisation. 



In the production of the metamorphic differentiation products, such as the chlorite 

 clot, we picture solid diffusion providing the means of molecular supply ; but there is 

 still some reason why all the chlorite molecules should converge to one centre. In the 

 formation of the zoned nodules of felspar, sphene, and magnetite in the amphibolite 

 No. 143, there must be some reason other than Riecke's principle why there should be 

 such an orderly arrangement. There must be some driving force which results first 

 in an attraction of the magnetite molecules to produce the magnetite nucleus, and 

 secondly in an attraction of the felspar molecules to give the felspar zone. Contempo- 

 raneously there is an outward diffusion of the ferromagnesian minerals. The production 



" The Segregatory and Migatory Habit of Solids in Alloys and in Steels below Critical Points," J. E. Stead, Journ. 

 Soo. Chem. Ind., 1903, p. 340. 



