XIX AUGITE-ANDESITES 273 



vicinity of the isolated hills and mountains of acid andesite, as 

 around Vatu Kaisia ; whilst they may enter into the formation of 

 the low basaltic plains as in the region west and south of the 

 Ndreketi River. They are, however, limited in their extent and 

 occurrence. From the large amount of glass in the groundmass, 

 they may be inferred to belong to flows formed under different 

 conditions from those under which the great basaltic plateaux were 

 formed, where the rock contains but scanty interstitial glass. 



In the slide they show the large plagioclase phenocrysts 

 together with a few small plates of ophitic augite in a groundmass 

 displaying in an abundant smoky glass a loose plexus of long 

 stout lathe-like plagioclase prisms partly wrapped around by lesser 

 augites. . . . The plagioclase phenocrysts, which attain a size of 

 4 to 6 mm., give lamellar extinctions of basic andesine (20° — 27°) 

 and show concentric zone-lines with transmitted light. They often 

 polarise in brilliant colours and are extensively cross-macled. 

 They contain usually abundant inclusions of the magma some- 

 times arranged zone-wise, and are frequently eroded. . . Non- 

 ophitic pyroxene phenocrysts are uncommon. In the slide occur 

 one or two small " plates," i to 2 mm. in size, of ophitic pale- 

 brown augite, and a number of lesser augites, '2 to 3 mm. in size, 

 which in part wrap around the felspar-lathes and by their aggrega- 

 tion form imperfect ophitic " plates." . . . The long stout felspar- 

 lathes, which are on the average '3 to -45 mm. in length, give 

 lamellar extinctions of 15° to 20° (medium andesine) .... The 

 copious smoky glass is rendered partially opaque by the abundant 

 development of rods and skeletal crystals of magnetite, and shows 

 the fibrous devitrification arising from the formation of incipient 

 microliths. In some rocks there appear in the smoky glass brown- 

 ish-yellow patches of the residual magma which under the micro- 

 scope cannot be distinguished from palagonite. 



All but one of the specimens belong to the species where the 

 felspar-lathes average over -3 mm. in length. 



B. NON-PORPHYRITIC SUB-GENUS 



Description. — Blackish-brown semi-ophitic rocks, sp. gr. 

 274 — 277, frequently of doleritic texture and showing a few small 

 macroscopic plagioclase phenocrysts. They are sometimes vesi- 

 cular, and form old flows in a few localities, as in the vicinity of 

 Natua in the eastern part of the Ndreketi plains and in the coast 

 district between Lekutu and Wailea Bay. 



T 



