FLXE-GEAINED BASALTS. 439 



labradorite prisms are more nearly parallel, with fluidal arrangement (PL 

 XXXIV, fig. 4). Some of the phenocrysts of labradorite-bytownite are 

 almost free from polysynthetic twinning, having but two or three parts. 

 Others (597, 612, 1139) are nearly the same, with small grains or crystals. 

 And still others (593, 610, 611, 636, 1138, 1141) are like the last, but con- 

 tain more augite than feldspar. 



VERY FIISE-GRAINED BASALTS WITH MINUTE PHENOCRYSTS. 



In the still finer-grained modifications of these basalts, which are very 

 numerous, the lath-shaped or prismatic labradorite crystals range in size 

 from about 1 mm. long to extremely minute microlites. They are in a mass 

 of magnetite grains and minute augite grains or crystals, sometimes with 

 brown glass base, as in the coarser-grained forms, some resembling the 

 foregoing rocks in microstructure (581, 616, 626, 662, 1140). The average 

 length of the labradorite prisms is 0.06 mm. Even in thin section they are 

 very dark gray. When still less crystalline they are darker, being nearly 

 black in thin section. There is more microlitic glass base, which is crowded 

 with grains of magnetite, some forming skeleton aggregates of the usual 

 stellate or cruciform shapes (619, 637, 653, 660). One variety from beneath 

 the rhyolite sheet on Mount Everts (621) has an almost panidiomorphic 

 structure, the augites being in minute prisms. Olivine is scarce. 



A modification of the fine-grained structure with feldspar prisms of 

 nearly uniform size is one in which many small prisms of labradorite are 

 in a groundmass of minute grains of magnetite and augite, about 0.01mm. 

 in diameter, and olivine also forms small phenocrysts (594, 607, 609, 638, 639, 

 666). The finest-grained form consists of a groundmass of grains of mag- 

 netite and augite 0.004 mm. in diameter, and very minute feldspars, with 

 many small phenocrysts of labradorite and olivine, about 0.5 mm. in length 

 and smaller (586, 587, 599, 1796). 



In all of these 69 recent basalts, with the single exception of the few 

 almost microscopic augites in a basalt on Gardiner River, south of Bunsen 

 Peak (1796), no augite phenocrysts occur. The most usual ones are labra- 

 dorite, and less often olivine. In most cases there are no distinct, mega- 

 scopic phenocrysts. Moreover, none of these basalts possess andesitic 

 structure. If more coarsely crystallized, they would probably have all be- 

 come ophitic. 



