SHOSHONITE OF TWO OCEAN PASS. 343 



magnetite. There is no olivine, and it is this fact which chiefly distinguishes 

 it from the other varieties of these rocks. 



Another dike (1318) in the same place resembles the one just described 

 in mineral composition, except that the only phenocrysts are small augites. 

 Two other dike rocks in this vicinity (1344, 1354) are like these in mineral 

 composition, with phenocrysts of labradorite and augite, but none of 

 olivine. In one case (1344) the microlites of plagioclase are surrounded 

 by orthoclase margins. 



The rock of the seventh analysis (1715) is the uppermost of the five 

 surficial lava sheets that overlie one another at Two Ocean Pass. It is 

 dark gray, with a waxy luster, and carries scattered phenocrysts of feldspar 

 and serpentinized olivine. In thin section the groundmass is seen to be 

 holocrystalline, and is composed of orthoclase crystals, both idiomorphic 

 and allotriomorphic, with much magnetite and augite and some chlorite or 

 serpentine, with red-brown biotite and hair-like needles of apatite. There 

 are comparatively large but microscopic dusted apatites among the pheno- 

 crysts, showing this mineral in two generations. The large apatites are 

 comparatively abundant, and were in part inclosed in the olivine, which is 

 wholly serpentinized. The feldspar phenocrysts are labradorite, with highly 

 developed polvsynthetic twinning. Several crystals are clustered together 

 into groups. In the rock of the second sheet (1716) the feldspar pheno- 

 crysts are labradorite-bytownite, being decidedly basic, with high extinc- 

 tion angles and relatively strong double refraction. Rectangular inclusions, 

 probably feldspar, are numerous; also considerable magnetite and augite, 

 and some serpentine. The groundmass is like that of the top sheet, except 

 that the orthoclase crystals sometimes have a small nucleus of lime-soda, 

 feldspar. These nuclei are sometimes colored green from serpentine, when 

 they are easily confounded with partly altered pyroxene. In other respects 

 the rock is like the uppermost sheet. The rock of one of these sheets 

 (1724) on the north side of Pacific Creek, 2 miles west of Two Ocean Pass, 

 is like the one last described, with somewhat larger phenocrysts of labra- 

 dorite, 5 mm. long. They have the same character and inclusions as the 

 labradorite phenocrysts of the last-mentioned rock. The olivines are 

 entirely altered to serpentine. The groundmass has the same composition 

 and structure. Some serpentine in the groundmass occupying spaces 

 between idiomorphic orthoclases may replace small interstitial bits of glass; 



