XVIII OLIVINE-BASALTS 



257 



4 mm. in size and in some rocks are less than i mm. They 

 are usually more or less serpentinised and hematised at the borders 

 and in the cracks ; but sometimes they are almost fresh and 

 present regular outlines. In some rocks the olivine also occurs as 

 grains (-3 mm.) in the groundmass. When the phenocrysts of 

 olivine have blackish borders they are surrounded by a halo, as 

 though the crystal had attracted the magnetite from the groundmass 

 immediately around. The plagioclase phenocrysts vary from i 

 to 3 mm. in size. They often contain abundant magma- 

 inclusions and give lamellar extinctions of 15" to 25° (basic ande- 

 sine). In some rocks they are rudely parallel. The pyroxene 

 phenocrysts, which are of pale brownish-yellow augite giving ex- 

 tinctions of over 30°, do not usually exceed 3 mm. They 

 present regular octagonal cross-sections and sometimes display 

 lamellar twinning. Occasionally there is a suspicion of intergrowth 

 with rhombic pyroxene. The felspar-lathes, which according to 

 the species' vary much in length, at times show a few lamellae. The 

 augite grains of the groundmass are abundant and are as a rule 

 about '02 mm. in size ; but in some rocks they are larger and in 

 others smaller. 



This genus may be divided into two sub-genera, the porphyritic 

 sub-genus where the felspar phenocrysts are larger than 3 mm., and 

 the non-porphyritic where they are smaller. All four species, as 

 indicated by the length of the felspar-lathes, are represented. 



14. Genus of the Olivine-Basalts 

 Formula. — Oliv, matr, cop, flu, gran, phen, opac. 



Characters. — Olivine abundant. Felspars of the groundmass 

 in flow-arrangement. Pyroxene of the groundmass granular. 

 Opaque plagioclase phenocrysts. 



Description. — Dark grey rocks, with sp. gr. 2-9 to 2^93, 

 showing phenocrysts of olivine and pyroxene with opaque whitish 

 phenocrysts of plagioclase in a groundmass of felspar lathes, 

 pyroxene grains, and magnetite, with occasional fine olivine. The 

 olivine phenocrysts, which are sometimes 5 or 6 mm. in size, are 

 often deeply eroded. They are at times so extensively hematised 

 along the schiller-planes that they appear like brown mica. The 

 plagioclase phenocrysts owe their opacity in part to their consisting 

 of an aggregate of lesser crystals which are clear and glassy and 

 give lamellar extinctions of 20° to 30° (andesine labradorite). 

 They do not usually exceed 3 mm. and are sometimes scanty. 



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