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PLUTONIC ROCKS 



249 



Fourth Order of the Hypersthene-Augite Andesites. 



Characters. — Groundmass presenting a rudely granular 

 appearance or blurred mosaic (microfelsitic). 



Formula. — Hyperstk.-aug., matr.,fels. 



Remarks. — My sliced specimens (five) are too few for the 

 elaboration of this order to which the classification employed for 

 the other orders is scarcely applicable. This is due to the partial 

 decomposition or imperfect development of the pyroxene of the 

 groundmass. The general characters of the order are given 

 on p. 291. 



The Plutonic Rocks. 



These rocks are very infrequent and are for the most part 

 hypersthene-gabbros or norites, with a few representatives of 

 diorites^ without pyroxene. True plutonic rocks did not come 

 under my observation in the western half of the island (west of 

 Larabasa and Savu-savu), those of Mount Thoka-singa in the 

 Ndrandramea district making the nearest approach (see p. 302). 

 The localities in which they were found are below enumerated : — 



1 I apply the term " diorite " to granitoid rocks formed entirely of plagioclase 

 and hornblende. 



