xxi ACID ANDESITES 



301 



the hill, are represented by pseudomorphs in various stages of 

 dispersion, so that this character is likely to be overlooked. The 

 pyroxene phenocrysts are in most cases of the pale yellow feebly 

 pleochroic rhombic type, but they may present intergrowths of 

 both the monoclinic and rhombic forms. These are light grey 

 rocks with a specific gravity increasing as one descends from the 

 summit, where it is about 2'5, to the base where it is 27, a change 

 corresponding with increase of the ferro-magnesian minerals and 

 with the more crystalline structure of the groundmass. 



The second species, where the " grain " of the mosaic is between 

 •01 and "02 mm., is represented by the Vatu Kaisia rock and by 

 that exposed in the opposite side of the gorge. They are granitoid 

 in appearance and have a specific gravity of 2 "68 to 271. Large 

 porphyritic crystals of pyroxene and hornblende, the last some- 

 times 7 mm. in length, occur in a dark grey base. In the slide 

 these phenocrysts together with those of plagioclase are displayed 

 in a somewhat scanty holo-crystalline groundmass, where the 

 "grain " of the mosaic averages - oi2 mm. The plagioclase pheno- 

 crysts are zoned, and give in different crystals extinctions in some 

 cases of oligoclase-andesine (io° — 12 ) and in others of andesine- 

 labradorite (25 — 30 ). The hornblende phenocrysts in their 

 pseudomorphism illustrate the various stages of the process of 

 conversion into fine granular and prismatic pyroxene. The least 

 altered crystals have dark resorption borders and are at times 

 deeply corroded. The pyroxene phenocrysts are for the most 

 part rhombic ; but intergrowths with the monoclinic form occur. 

 The pyroxene of the groundmass consists for the most part of 

 small rhombic prisms averaging '05 mm. in length. 



The third species, where the groundmass may be described as a 

 coarse mosaic with a " grain " between 02 and "03 mm., is repre- 

 sented by the rocks of the peak of Na Raro and of Mount Thoka- 

 singa in the Ndrandramea district. 



The Na Raro rocks are light grey with a specific gravity in the 

 unweathered state of about 2 - 6, and display macroscopic crystals 

 of glassy plagioclase and hornblende. In the slide they exhibit 

 tabular phenocrysts of the plagioclase, together with dark pseudo- 

 morphs after hornblende, in a coarsely felsitic groundmass (grain 

 •022 mm.) where a little very fine prismatic pyroxene, apparently 

 rhombic, occurs. There is also a little altered interstitial glass. 

 The plagioclase phenocrysts are zoned and give extinctions of 15 

 to 25° (acid and basic andesine), whilst they often show magma 

 inclusions. An interesting feature of these rocks is concerned 



