CHAPTER XIII. 



THE GENERAL PROBLEM OF TRANSFERENCE OF MATERIAL DURING 



METAMORPHISM. 



There is no fundamental difference between the processes which have been termed 

 metamorphic diffusion and metamorphic differentiation. The two terms have been 

 introduced for convenience. Metamorphic diffusion merely involves a migration of 

 material in the solid rock during the recrystallisation, and has been studied along pre- 

 existing junctions. Metamorphic differentiation requires, in addition to a migration, 

 a segregation of migrated molecules. Metamorphic diffusion of some constituents is 

 necessary to bring about metamorphic differentiation, and in this way a metamorphic 

 differentiation product is also a metamorphic diffusion product ; but the converse is 

 not true. Both processes are probably governed by the same fundamental principles, 

 and both involve the wide problem of the general transfer of material during meta- 

 morphism. The principle asserted in this problem is one that has not been accorded 

 general acceptance chiefly on account of the paucity of sure evidence and the possibility 

 of alternative hypotheses. 



EVIDENCE OF MIGRATION IN GEOLOGICAL LITERATURE. 



Chemical changes in total composition during metamorphism has been recently 

 argued by Leith and Mead,* and each chemical change requires a migration. These 

 authors look upon amphibolite as an end product of the metamorphism of marble. 

 Though we see reason to question this view, we think that the phenomena described 

 by Adams and Barlow f at the junction of a crystalline limestone and an amphibolite 

 provide sound evidence of a transfer of material during metamorphism. Further 

 examples are quoted by Leith and Mead. W. S. Bayley has provided the record in the 

 Menominee District of Michigan that " at some places the dolomites at their contact with 

 their overlying iron formation have been entirely changed from their original condition 

 and are now represented by talc and serpentine."! Such a change would involve an 

 important change of composition. The transformation of quartzite into sericite schist 

 is a change that has been substantiated by Truemann, but, so long as the authors ascribe 

 the removal to the agency of solutions, it cannot be considered as evidence of the 

 molecular transfer of material in the solid state during dynamo metamorphism. Strong 

 evidence has been put forward by G. H. Williams, who has traced a gabbro into a 



* " Metamorphic Studies," C. K. Leith and W. J. Mead, Journ. Geol., vol. XXIII., 1915, p. 602. 

 f Op. cit., Adams and Barlow, p. 87. 



J " Menominee Iron Bearing District of Michigan," W. S. Bayley. Mon. 46, U.S. Geol. Survey, 1904, p. 221. 

 " The Greenstone Schist Areas of the Menominee and Marquette Regions of Michigan." Bull. 62, U.S. Geol. Surv., 

 1890, p. 76. 



