410 THE JOURNAL OF (GEOLOGY. 
are numerous bodies of a dense fine-grained gray lava, which 
usually weathers with a slaty fracture, the apparent cleavage 
being often vertical. The rock is composed of plagioclase, augite, 
a slightly pleochroic rhombic pyroxene, and grains of magnetite. 
The feldspar, augite, and rhombic pyroxene are in the form of 
minute elongated prisms or laths, and this is true of the rock at 
widely separated localities, and the laths of all are nearly of the 
same size. About half a gram of No. 661 powdered and treated 
with HCl by George Steiger yielded no gelatinous silica, but 
nevertheless there appears to be some glass present. 
Three analyses are given in the table of this pyroxene- 
andesite. The analysis of No. 1829 Cascade Range collection* 
is “Of a “specimen collected’ by Mr Diller, from” the west 
summit of Crater Peak (Lassen Peak atlas sheet); No. 209 Sierra 
Nevada is from the high point one and one-fourth miles north- 
east of Goodyear’s Bar; and No. 661 is from Franklin Hill ( Bid- 
‘well Bar atlas sheet). The analyses of 1829, and 209 are by Dr. 
Hillebrand, and the partial analysis of 661 is by Dr. Stokes. In 
the vicinity of Poker Flat are numerous dikes of another fine-_ 
erained andesite very similar in general appearance to the variety 
just described. It differs in being frequently columnar, and 
microscopically in containing a little hornblende and in the feld- 
spars being less uniform in size and shape. At one point on the 
summit of the ridge south of Poker Flat a dike (No. 631 S. N.) 
of this rock cuts the hornblende-pyroxene-andesite-breccia. 
Another and larger dike cuts the pre-Tertiary rocks east of Poker 
Flat. It crosses Canyon Creek just east of the mouth of Illinois 
ravine, and to the southeast of Poker Flat forms a conical butte 
showing columnar structure. An analysis of the latter dike is 
given in the table (No. 262 S. N.). This andesite is therefore 
later than the hornblende-pyroxene-andesite-breccia. The exact 
relative age of the similar fine-grained andesite (Nos. 1829, 209 
and 661) is still in doubt, but it is thought to be of about the 
same age as the andesite (Nos. 631 and 262) just described. 
The rock called quartz-andesite (No. 626 S. N.) in the table 
* Bull. No. 60, U. S. Geol. Survey, p. 157, No. 19. 
