XVIII OLIVINE-BASALTS 265 



Two species are represented in this collection : — 

 {a) with felspar-lathes '2 — "3 mm. 



(^) „ » » '3— "5 ,. 



44. Genus of the Olivine-Basalts 



Formula. — Oliv, matr, pauc, flu, prismy non-phen,parv. 



Characters. — Olivine scanty. In the groundmass the 

 felspar-lathes are in flow-arrangement and the pyroxene is in great 

 part prismatic. There are no phenocrysts of plagioclase ; but there 

 are a few small phenocrysts of olivine and sometimes of pyroxene, 

 less than 2 mm. in size. 



Description. — A dark grey compactish rock ; sp. gr. 2*9 ; 

 showing a little macroscopic olivine ; forming a dyke in the tuffs 

 on the summit of the hill of Vatui (p. 54). 



In the section it exhibits a few phenocrysts of olivine (more or 

 less serpentinised) and of augite in a groundmass formed of stout 

 augite prisms and small augite granules with felspar-lathes, mag- 

 netite, and a little devitrified yellowish interstitial glass. The 

 augite prisms and the felspar-lathes are in flow-arrangement. . . . 

 The pyroxene phenocrysts, which are pale brown and give ex- 

 tinctions of over 30° from the single cleavage-lines, may be aggre- 

 gates of five or six smaller crystals or single crystals presenting 

 sometimes lamellar twinning. The first are doubtless formed in 

 situ. The second though showing regular outlines may have a 

 nucleus giving a different extinction and possessing eroded margins. 

 The stout augite prisms of the groundmass, which are occasionally 

 twinned, have an average length of '2 to "3 mm. and give angles of 

 extinction with the long axis of 30° to 40°. The felspar-lathes 

 average only 'oy mm. in length and afford extinctions, when 

 untwinned, of 18° to 24° (acid labradorite). 



The only species represented is that where the average length 

 of the felspar-lathes is between -02 and "i mm. 



