THE METAMORPHIC ROCKS OF ADELIE LAND. 8TILLWELL. 201 



green-stone schist (sericite chlorite schist), and the alteration in composition is proved 

 by chemical analyses. He correlates the chemical changes between fresh gabbro, 

 8 aussuritised gabbro, and greenstone schist with varying amounts of chloritisation, 

 sericitisation, and carbonation. 



Further evidence still is claimed in the theory advanced by Leith and Mead that there 

 is a convergence under metamorphic conditions towards definite mineralogical types, 

 chiefly hornblende, chlorite, or mica. If such a conception is warranted it does provide 

 extra evidence. The unpublished text book referred to in their paper is now available,* 

 but no detailed petrological study is forthcoming of the instances in which these types 

 are produced. The theory depends largely on the comparison of groups of analyses of 

 sericite schists, chlorite schists, and hornblende schists, which have been divorced from 

 all field associations and from questions of origin. The theory at present remains a 

 mere speculation until some knowledge is gained of the fate of that part of the chemical 

 composition of the rock which must be rejected in order to produce hornblende, mica, 

 or chlorite rocks. The theory further neglects to take account of those types of meta- 

 morphic products which are produced under metamorphic conditions incompatible 

 with the existence of chlorite, hornblende, or mica, e.g., pyroxene gneisses or any rock 

 that is formed under the conditions of Grubenmann's kata zone where the normal 

 structure is massive not schistose. 



It may be suggested that the " convergence " theory is an incomplete expression 

 of the theory of metamorphic differentiation, because it has been shown that hornblende 

 rock, chlorite rock, and biotite-hornblende rock are formed by metamorphic differentia- 

 tion. If it be subsequently found that metamorphic differentiation can only occur under 

 those metamorphic conditions which tend to produce hornblende, chlorite, or mica, 

 then there will be a closer agreement between the theories than is anticipated. As, 

 however, we have instanced a large garnet crystal as a minute differentiation we must 

 at least suppose that differentiation can occur under garnet-forming conditions which 

 are normally those of the kata zone. 



Some evidence, therefore, has been culled from the geological literature by Leith 

 and Mead to establish the theory that some transference of material occurs during 

 metamorphism. On the other hand a wealth of evidence can be obtained to prove 

 that in many instances no important change has occurred. This, indeed, is the basic 

 principle upon which Grubenmann has built his classification of schists. Hence the 

 degree and range of the transference must necessarily be limited. No more than this 

 is implied in the theory of metamorphic diffusion. 



If transference of material occurs across a pre-existing junction of two rock types 

 the junction will be replaced by a transition which we call a metamorphic diffusion 

 product. Many examples of destroyed junctions have been described between igneous 

 and sedimentary rocks, but in most cases it has been ascribed to assimilation of the 



" MeUmorphic Geology," C. K. Leith and W. J. Mewl. New York. 1016. 



