part 2] or southeen eyee PENiifsuLA. 109 



Rocks which represent a higher grade of metamorphism than 

 the common type of amphibolite are observed among the garnet- 

 amphibolites (or, more correctly termed, ' hornblende-eclogites '). 

 These rocks have been derived from true pyroxenic eclogites, and 

 the pyroxene which they now contain is not the original p3"roxene 

 of the igneous rock, but this mineral recrystallized. 



L. Hezner^ andU. Grubenmann ^ have shown that the garnet of 

 the eclogites is to be regarded as the result of the interaction of the 

 original pyroxene with plagioclase, or in some cases of olivine with 

 that mineral, and the equation to represent this change need not 

 be represented here. It is sufficient to note that, in the former 

 case, an essential bj^e-product is silica, and appears as quartz. 



The true pyroxenic eclogites are characteristic rocks of 

 Grubenmann's ' Kata-zone,' and the hornblende-eclogites have 

 developed under conditions in which shearing-stress is more in 

 action. There is, however, another source froin which the garnet 

 of these rocks may be derived, which involves a rearrangement of 

 the pyroxene-molecule without the interaction of the felspar- 

 molecule. This may be concisely stated in the following 

 equation : — 



iMg?i!sio!' 1 =CaMg-,Al,SiA. + C^-l) CaMgSiA. 



Of this reaction there is fairly conclusive proof in certain 

 metamorphosed dolerites which will be dealt with later, for the 

 metamorphism of these rocks is accompanied by reaction -rims of 

 the individual minerals, but wdth more or less preservation of the 

 original igneous textures, enabling us to see the transformations 

 in progress. It is, therefore, possible that in the j^yroxene- 

 eclogites some of the garnet has originated in this manner. 



In the hornblende-eclogites of the Flmders Series, the formation 

 of amphibole from the pyroxene of the true eclogite has evidently 

 proceeded at a greater rate than the garnet degradation ; or. at 

 least, garnet is more stable under the conditions that were then 

 prevailing. Thus the garnet is now only showing signs of in- 

 stability, as represented by the diablastic intergrowths of horn- 

 blende and plagioclase Avhich fringe its borders. These, by their 

 manner of development, show that the jDrocess is one of degrada- 

 tion rather than of formation of garnet, and this change may be 

 represented as follows : — 



It has already been noted that the presence of a subordinate 

 amount of quartz as a general constituent of amphibolites is to be 

 regarded as the normal result of metamorphism. 



There are, however, present in the Flinders Series amphibolites 

 (of which a petrographic description has been given) that con- 

 tain a greater percentage of quartz than can be ascribed to any 



1 Min. Petr. Mitth. vol. xxii (1903) pp. 473, 505. 

 - 'Die Kristallinen Schiei'ei' ' 1910, p. 53. 



e2 



