X907.] CRANDALL— SAN FRANCISCO PENINSULA. 23 



the two minerals are iridistingiiishable. In polarized light the epi- 

 dote shows grey colors of the first order but the clino-zoisite shows 

 upper first and second order colors. These minerals are plainly 

 secondary but whether they are derived from the alteration of plagio- 

 clase is not apparent. When these veins are cut at right angles 

 to the columnar length of the epidote crystals, minute cross sections 

 are seen. These aggregates are composed of such small crystals 

 that the compensating extinction gives the whole section a grey 

 appearance that does not change in polarized light, with a low 

 power objective. 



Auiygdaloidal Diabase. — From the north side of Visitation Valley 

 two outcrops in place give specimens that, though badly weathered, 

 show hornblende and feldspars. One of these rocks shows the horn- 

 blende going to chlorite in most of the slides. The chlorite is 

 green and yellow, with a similar color in polarized light. Titanite 

 is present in this rock in rounded grains showing rough relief and 

 irregular cleavage cracks. In polarized light the interference colors 

 are third order and higher, with an extinction angle that is not 

 consistent. Near the south side of the entrance to Visitation val- 

 ley is an iron stained rock that looks like a weathered conglomerate 

 in the field. Sections of this show it to be an amygdaloidal diabase. 

 The texture is ophitic in the lath-shaped feldspars in a groundmass 

 of allotriomorphic hornblende. A few large crystals of feldspar 

 occur, none of which show twinning, though most of the small ones 

 do. The extinction angles obtained were i8°, placing them near 

 the acid end of labradorites. The hornblende has large cavities 

 filled with calcite mainly, but chlorite and some iron mineral are 

 present also. The chlorite is green but non-pleochroic ; in polarized 

 light it shows blue and brown. The iron mineral is a dark reddish 

 brown with a slight metallic lustre. 



Hornblende Diorite. — A specimen of rock from the southwest 

 side of the San Bruno mountains shows hornblende and plagioclase 

 but without ophitic texture. The hornblende and plagioclase are 

 the same as those already described, but it contains two new secon- 

 dary minerals, spinel and titanite. 



Spinel is present in large quantities, in rounded irregular grains 

 with high relief and rough surface. Being an isotropic mineral 

 it remains dark under cross nicols. 



