xvm OLI VINE-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 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*91. 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. 2-9. It 

 displays abundant small phenocrysts of olivine, all less than a 



