XVIII OLIVINE-BASALTS 263 



length '15 — •21 mm. The sp. gr. is usually between 2'8y and 2*9 1. 

 Though rarely porphyritic, such rocks display to the eye a few 

 small scattered glassy phenocrysts of plagioclase and an occasional 

 grain of olivine. It is to this type of the genus that the following 

 description applies. 



In the thin sections they display small plagioclase phenocrysts, 

 with a few of olivine and occasionally of pyroxene, in a ground- 

 mass where the flow-arrangement of the felspar-lathes is well 

 marked, the rest of the groundmass being made up of granular 

 augite with magnetite and generally a little residual glass. . . . The 

 plagioclase phenocrysts do not usually exceed 2 mm. in size and 

 contain magma inclusions. Two kinds are often indicated in the 

 same slide by the extinctions, namely, one of medium andesine 

 (17° — 22°), and the other of acid labradorite (28° — 33°). . . . The 

 pyroxene phenocrysts are of pale brown augite ; but they are small 

 (less than 2 mm.), scanty, and often absent when their place is 

 taken by microporphyritic augite, '2 mm. in size. . . . The olivine 

 phenocrysts are generally small. Though sometimes showing the 

 long hexagonal sections, they are often rounded and more or less 

 serpentinised. . . . The felspars of the groundmass present more 

 typical lathes than are to be observed in the non-parallel felspars of 

 the rocks of genus 25. The twin-lamellae, when present, are fewer ; 

 but give similar extinctions (15° — 21°) of medium andesine. . . . The 

 augite granules are, as a rule, "02 or '03 mm. in diameter ; but 

 occasional more prismatic forms occur, two or three times the 

 length, which give extinctions of over 30°. The magnetite is 

 abundant, and the scanty interstitial glass is green or brown and 

 displays fibrous devitrification. 



The following three species, as indicated by the length of the 

 felspar-lathes, are represented in my collection : 



(fl) -02 — -I mm. (d) -I — -2 mm. 



(c) '2 — "3 mm. 



38. Genus of the Olivine-Basalts 

 Formula. — OHv, matr, pauc, Jlu, gran, phen, opac. 



Characters. — Olivine scanty. In the groundmass the fel- 

 spar-lathes are in flow-arrangement and the pyroxene is granular. 

 Opaque plagioclase phenocrysts. 



Description. — Grey rocks, sp. gr. 278 to 2-83, showing small 

 opaque porphyritic crystals of plagioclase with a few phenocrysts 



