xviii OLIVINE-BASALTS 



255 



macHng. They give extinctions of acid labradorite (25° — 32°).. 

 Pyroxene-phenocrysts, when present, are scanty, pale brown, not 

 over 2 mm. in size, and give the large extinctions ( + 30°) of augite. 

 In their absence small augites ('2 mm.) are frequent. Fine 

 granules (-oi — -03 mm.) of similar augite, together with magnetite, 

 abound in the groundmass. The felspars of the groundmass are 

 fairly stout and lathe-like and show at times a few twin-lamellae 

 which give extinctions of 13° to 20° (medium andesine). 

 Species represented : 



(a) felspar lathes 'i — -2 mm. 

 (^) „ ., '2— -3 ,, 



3. Genus of the Olivine-Basalts 

 Formula. — Oliv, matr, cop, non-flu, gran, non-phen, magn. 

 Characters. — Abundant olivine. Felspar-lathes of the 



groundmass not in flow-arrangement. Pyroxene of the ground- 

 mass granular. No plagioclase phenocrysts. Large phenocrysts 

 of olivine and pyroxene over 2 mm. in size. 



Description. — A remarkable blackish-grey rock. Although 

 somewhat scoriaceous, it has a sp. gr. of 2'9i. It displays large 

 phenocrysts of olivine and pyroxene, 4 to 8 mm. in size, in a 

 coarse-textured groundmass of stout felspars, augite granules, 

 magnetite and a little glass. The olivine is extensively hematised. 

 The pyroxene phenocrysts are of brownish-yellow augite with 

 regular outlines and giving angles of extinction up to 40°. The 

 broad lamellar felspars of the groundmass, which are on the average 

 ■3 to '4 mm. long, give extinctions indicating both acid labradorite 

 (23° to 28°) and medium andesine (16° or 17°). The abundant 

 augite-grains average '05 mm. in size ; but the prism form occurs 

 at times. 



The species with felspar-lathes "3 to -5 mm. long is alone 

 represented. 



4. Genus of the Olivine-Basalts 

 Formula. — Oliv, matr, cop, non-flu, gran,non-phen, parv. 

 Characters. — Abundant olivine. In the groundmass the 



felspar-lathes are not in flow-arrangement and the pyroxene is 

 granular. No plagioclase phenocrysts. Small phenocrysts of 

 olivine and occasionally pyroxene under 2 mm. 



Description. — A grey coarse-grained rock. Sp. gr. 29. It 

 displays abundant small phenocrysts of olivine, all less than a 



