ABSAEOKITE OF TWO OCEAN PASS. 337 



and needles of apatite complete the resemblance between this rock and that 

 just referred to. It appears to be an intermediate variety between the 

 latter and certain orthoclase-bearing basaltic rocks from the head of Conant 

 Creek, to be described presently (p. 338). Its appearance in this section is 

 shown in PL XXXVI, fig. 2. 



Another modification of the lava at Signal Point (1617) contains much 

 orthoclase in the groundmass, no isotropic mineral, but considerable reddish 

 brown mica, which appears to be secondary and accompanies chlorite or 

 serpentine. It is sometimes sharply idiomorphic, and has a small axial 

 angle. There is also some red oxide of iron. Another variety has a nearly 

 opaque groundmass, with reddish-brown color, carrying phenocrysts of 

 augite and olivine like those in the rocks associated with it. In A T ery thin 

 section the groundmass is seen to contain much iron oxide and feldspathic 

 mineral, partly in lath-shaped microlites, altered to faintly doubly refract- 

 ing substance. The rock is undoubtedly a somewhat altered, opaque, 

 scoriaceous modification of this kind of rock. 



The massive lava flows at Two Ocean Pass belong in part to this same 

 class of rocks, but vary somewhat in composition in the different sheets, 

 which directly overlie one another. The rock most closely resembling those 

 just described forms the second sheet from the bottom (1720, 1721). Its 

 chemical composition is shown in analysis 6, p. 329. It contains similar 

 phenocrysts of augite and olivine, with none of feldspar. The feldspar of 

 the groundmass is almost wholly orthoclase, sometimes in simple prismatic 

 crystal, sometimes in Carlsbad twins. It is nearly idiomorphic. There is 

 a small amount of isotropic mineral as interstitial filling between orthoclase 

 prisms. It has a slightly lower index of refraction. It may possibly be 

 glass. The groundmass also contains augite, magnetite, serpentine, red- 

 brown biotite, and needles of apatite. No lime-soda feldspar was observed. 

 The groundmass is holocrystalline, the average size of the component ortho- 

 clase being about 0.07 mm. wide by 0.14 mm. long. Chemically it differs 

 from the most typical absarokite by being higher in alumina and lower in 

 magnesia, approaching shoshonite in chemical composition, with which rock 

 it is associated in the field. 



The scoriaceous portion of this rock (1721) is almost opaque in thin 

 section, and in very thin edges is seen to consist of minute crystals of mag- 

 netite, pyroxene, and a feldspathic mineral whose character is not recogniz- 



MON XXXII, PT II 22 



