THE METAMORPHIC ROCKS OF ADELIE LAND. -STILL WELL. 205 



finely granular sorbite and laminated pearlite are successively obtained. Further 

 heating causes the pearlitic laminae to contract, producing beaded forms and ultimately 

 the carbide segregates into coarse masses. 



The segregation of this constituent, iron carbide, in steels seems to be in some 

 measure analogous to the segregation of chlorite in the amphibolite rocks at Cape Denison. 

 Though the amphibolite cannot be looked upon as a solid solution, yet in each case there 

 is the segregation of a single constituent within a solid mass. In both cases it is an 

 adjustment of physical equilibrium, not chemical equilibrium. It is, of course, true 

 that the segregated mass of chlorite is many times larger than the segregated mass of 

 carbide in steel, but the former appears in the rocks of the greatest geological antiquity, 

 while the latter is developed in a few hours in the laboratory. In the other cases of 

 metamorphic differentiation where more than one constituent has been segregated, 

 the analogy is still suggestive of a mental picture whereby the process can go on. 



" In alloys," says the report, " which contain two solid solutions in equilibrium 

 with one another, such as the <* ft alloys of copper and zinc, the structure becomes 

 coarser when the alloys are heated at a temperature at which diffusion takes place. 

 The increase in size of crystals is equally pronounced when only a single constituent 

 a pure metal or a solid solution is present. The growth of the ferrite grains in soft 

 steel at 700-720 is extremely rapid, and the process can be conveniently watched in 

 other instances. It is always the larger crystals that absorb the smaller." 



' Whether the metal in which such recrystallisation takes place is homogeneous 

 or heterogeneous, diffusion must occur in order that rearrangement may come about. 

 The effect has been explained by the principle that small crystals have a greater 

 solubility than large, so that if small and large crystals of the same substance are both 

 in the presence of a solvent, solution and redeposition tend to go on until only crystals 

 above a certain size are present. This has been verified for the case of calcium sulphate 

 in water. A thermo-dynamical explanation has also been given of the principle that 

 the bounding surface between a crystal and its saturated solution tends to become a 

 minimum, so that equilibrium is only finally reached when all the small crystals have 

 united to form a single crystal." 



" The principle of differing solubility is rejected as an explanation by G. Tammann, 

 who assumes that the surface tension, which is less than the forces producing rigidity in 

 a crystal at the ordinary temperature, may become much more considerable with increase 

 of temperature. When the surface tension exceeds the opposing forces, two crystals 

 unite as two drops of fluid do. The hypothesis is applied to explain the recrystallisation 

 of strained minerals." 



This increase in grain size in metallic alloys seems to be closely analogous to the 

 increase in grain size in certain metamorphic rocks. If solid diffusion is an operating 

 factor in one case, it must occur in the other. At Cape Gray the first metamorphic 

 product derived from a dolerite has a very fine granulitic structure, with average absolute 



