THE METAMORPHIC ROCKS OF ADELIE LAND. 8TILLWELL. 197 



been considered as a high-temperature and high-pressure alteration form of gabbro,* 

 and further, that it is a product of the kata zone of metamorphism in which Grubenmann 

 has recorded garnet as a common mineral. f 



The hypothesis has only been put forward, so far, in a short preliminary paper 

 in a general manner. The absence of detail and of references makes it difficult to arrive 

 at a just estimate of its worth. The shell is supposed to be normally solid, and only 

 becomes liquid on release of pressure ; yet the constituents in this zone are, in a general 

 way, spoken of as " crystallising out." The phrase, " crystallising out," is generally 

 used in reference to magmas and solutions, but it may be applied to solid solutions. 

 Fermor does not indicate that he is referring to solid solutioas. 



In the infraplutonic zone the formation of garnet occurs in those reactions and 

 rearrangements which are accompanied by reduction of volume and absorption of heat ; 

 and he points out that the formation of garnet from other minerals, such as pyroxenes, 

 olivines, and iron ores, is always accompanied by decrease in volume. But he does not 

 offer any explanation as to how the pyroxene, olivine, and iron ore happen to reach 

 the infraplutonic zone. If we are to suppose that some pre-existing rock is buried 

 by earth movements to such a depth that it reaches the infraplutonic zone, we are 

 merely imposing a set of metamorphic conditions upon the rock. In this case the infra- 

 plutonic zone is indistinguishable from a metamorphic zone defined by the particular 

 set of conditions. If melting should follow the exit of the rock from the infraplutonic 

 zone the rock will assume the characters of a normal eruptive rock. If melting does 

 not occur with release of pressure, and it has not occurred in the garnet rocks that I have 

 studied, a normal metamorphic rock would appear to result. 



The difference between the infraplutonic zone and the kata zone of metamorphism 

 has not been considered by Fermor. If there is a similarity we point out that the infra- 

 plutonic conception involves the worst feature of the conception of metamorphic zones 

 viz., that of depth. A metamorphic zone can only be adequately defined by a set of 

 physico-chemical conditions, and not by varying depths in the earth's crust. 



When a plutonic rock forms from its magma we date its existence as a unit from the 

 time of its consolidation. The petrologist must, at present, be content to leave open 

 the questions concerning the origin of the magmas, because " cosmogony can afford 

 no firm foundation for a priori reasoning."f Perhaps we should do the same with the 

 products of the infraplutonic zone. Perhaps an infraplutonic zone product is meant 

 to be analogous to a plutonic zone product, differing only in the conditions of temperature 

 and pressure under which it forms. If this is so the infraplutonic conception does not 

 offer any explanation at all of the large number of garnet rocks which have formed 

 by recrystallisation in the solid state, and whose former igneous or sedimentary origin 

 can be traced. 



* " Ein Beitng zur Kerntnu der Eclogite und Amphibolite mit beaonderer benicksichtigung der Vorkommniane dw 

 Mittleren Otrtalea," Laura Hezner, published by Alfred Holder, Wien, 1903. 

 f Grubenmann, op. cit., vol. II., p. 83. 

 J " Natural Hutory of Igneous Rock," A. Barker, p. 4. 



