THE METAMORPHIC ROCK8 OF ADEL1E LAND. KTILLWELL. 203 



THE PROCESS OF MIGRATION. 



General remarks on the problem of transference of material during metamorphism 

 may be offered under the heads 



1. Solution. 



2. Solid Diffusion. 



3. Force of Crystallisation. 



The remarks are confined to instances of metamorphism unaccompanied by the 

 addition of material from any foreign source. The rocks during metamorphism have 

 remained throughout as a solid, self-contained mass. There has been no essential change 

 in chemical composition, and metamorphism is conceived to be merely a recrystallisation 

 caused by super-imposed physico-chemical conditions. We also do not consider instances 

 in which partial fusion is assumed, because we have not met with any examples in our 

 field study. 



1. Solution. 



Solution as a transferring agent is always the factor to which there is ready appeal, 

 and it is not easy to understand how the decrystallisation of a rock can proceed without 

 it. There is always a measurable quantity of water ( H 2 0) in rocks, and this is 

 present in capillaries and hollow spaces ; in addition there is the chemically combined 

 water ( -f H 2 0) which is freed at higher temperatures, and may become an active solvent. 

 Magmatic water may also be included when finding its way to the surface for the first 

 time. Thus there is always a minute amount of solvent present. 



There is no need to enter into the discussion of conditions that assist or prevent 

 solution. But it must be admitted here, that, provided that the metamorphic conditions 

 are such that water can circulate, solution in some places, with corresponding deposition 

 in other places, can be an agent in the transference of material in rocks which do not lose 

 their rigidity. This factor is, perhaps, more noticeable in the epi zone of metamorphism 

 which is more particularly characterised by the hydrous silicates. The presence of 

 minute fractures is possible in this zone, and these cracks provide more ready channels 

 of water percolation. In the production of amphibolites by the metamorphism of 

 dolerites under epi conditions, we occasionally find microscopic veins of scapolite, 

 lawsonite, calcite, and epidote, whose origin has been traced to large crystals of 

 saussuritised felspar. In such cases solution has been a direct agent in the transference 

 of material. 



Microscopical solution and deposition is implied in the principle called Riecke's 

 principle by Becke and Grubenmann. This principle is discussed by Johnston and 

 Niggli,* who point out that it depends on the thermodynamical fact that unequal pressure, 

 acting only on the solid phase, increases its vapour pressure and its solubility (in any 



" General Principles Underlying Metamorphic Processes," Journ. Geol. XXL, p. 603. 



