xvm OLIVINE-BASALTS 255 



macling. 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 ground mass are 

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

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

 Species represented : 



(a) felspar lathes t — '2 mm. 

 i b ) » », -2— "3 „ 



3. Genus of the Olivine-Basalts 



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



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*9 1. 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, noii-p hen, 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 



