TIIK UOdKK OF Til 10 WASIIOK D'STKKT. 49 



close-j^niwiiij^' l)mic-lies of grapes. In still dtlier slides nraiiis ot'tlie mineral 

 are distributed tliruiigli the gTouudiuass. 



Mineral constituents in detail. Tllls TOck alvVajS COlltaillS ])Or[)liy riticjll teld- 



spars. They are long, sharply rectilinear, ;nid without exception triclinic. 

 They give angles of extinction proper tu labradoi-ite. The laniella; are of 

 moderate width, and are often combined at the same time according to all 

 the connnon twinning laws. In nearly every slide they carry li(pii(l inclu- 

 sions, generally of vesicular shajjcs. The smaller feldspars form granitoid 

 grains of "secondary consolidation," and with the iron ores and more or less 

 augite, make up the groundmass. I have obser\'ed some of these smaller 

 feldspars which gave angles of extinction indicating a different species from 

 the larger crystals of first consolidation. The iron ore is in part magnet- 

 ite, and in part ilmenite, with the characteristic cleavage-lines and products 

 of decomjjosition. 



Quartz grains of unquestionably primitive character are occasionally 

 met with. These show an arrangement of j)articles of niiignetite, etc., 

 about their peripheries such as secondary quartzes never exhibit. Almost 

 all of them show fluid inclusions, the smaller ones with moving bub- 

 bles. I have observed none in which tlie li(juid appeared to be in the 

 spheroidal state, and the l)ubbles do not disappear at a temperature of 

 above 40° C; the fluid is therefore aqueous. I have met with no salt 

 cubes. Hornblende occurs spai'inglv, and is generally confined to closely- 

 limited areas. Where it is present great care is necessary in discriminating the 

 rock macroscopically from diorite Mica is rare, and is seen only in almost 

 indeterminably small particles, which might even be secondary. The 

 apatite is of the usual colorless variety. Not a single zircon was detected. 



Evidences of diabasitic character. TIlC micrOStrUCturC of this rOck StrOllgly SUg- 



gests that of some lavas, and I have sometimes been puzzled to say at the 

 first glance whether a particular slide was augite-andesite or diabase; but 

 the resemblance is superficial. As will be seen later, somewhat granular 

 augite-andesites occur in the district, l)nt they are exceptional. Here as 

 elsewhere the younger rock generally shows a microlitic groundmass, and 

 frequently a glass base. This is the case equally on the surface, and in the 

 Sutro Tunnel more than a thousand feet beneath the surface, Tlus diabii-^o now 



5 L 



