. 68 GEOLOGY. 
by sandstone strata reaches nearly to the sedimentary formations of the Great Basin, being 
separated from them by a narrow, intervening ridge of granite or slates. There is here a 
geological indication of a good pass, but the crest or ridge of granite was not explored. 
Williamson’s Pass may be considered as formed by a similar and parallel valley on the oppo- 
site side of the slate ridge and the granite which adjoins it. The presence of low hills, 
composed of sedimentary rocks, near the entrance to the pass has already been noted. Our 
route from the Cow camp to the pass was further south than before; and we travelled among 
the hills and not around them. They appear to be principally composed of light clays, sand, 
pebbles, and debris of granitic rocks; all being of a light color, and showing an abundance of 
pink feldspar. Gypsum, in thin seams, was also found in some of the lower strata. 
Breccia of volcanic rocks.—A narrow valley intervenes between the low hills and the volcanic 
or eruptive rocks, which form the lower ridge of the main Sierra. Before reaching the intrusive 
rock, I passed thick strata of breccia, composed of angular blocks of porphyritic volcanic rocks, 
of various colors, principally red, brown, and a chocolate color. These blocks were closely 
impacted, and varied in size from two or three inches to ten and twelve inches in diameter. 
The strata dip at an angle of 20°, inclining towards the northeast. 
Dyke of erupted rock.—The volcanic rock adjoining this breccia forms a considerable ridge, 
‚which is crossed by the trail at nearly a right angle. This rock presents a variety of characters 
and colors. A large portion of it is compact and dark-green, not unlike ordinary trap rock; 
other parts are porphyritic, being composed of a compact base of a chocolate-brown color, filled 
with minute white crystals of feldspar. The brown parts of the ridge are also more or less 
striped, and filled with long, parallel, and thin lines of different shades of colors, giving the mass 
a ribboned and veined appearance. Various shades of red were also observed in different places; 
and both the reddish and brown portions are porphyritic. The darker portions of this ridge 
were nearest to the breccia above described. The surfaces of fissures were more or less covered 
with a dark-green coating, which appeared like a crust of carbonate of copper, but does not 
contain a trace of that metal. Some of the fissures also contained imperfect crystallizations of 
a zeolitic mineral, in radiated and stellar forms—probably stilbite. Agates of various sizes and 
forms were also seen in the beds of the streams. 
It is very possible that this erupted ridge consists of more than one intrusion ; the differences 
found in the rock at different places may be regarded as an indication of it. The ridge, con- 
sidered as a whole, may be called a compact, fine-grained porphyry. It appears to have a 
width or thickness of nearly 2,000 feet, and to extend in a southwesterly direction, nearly 
parallel with the valley of the pass. 
The volcanic or eruptive rock, seen on the 22d, further east, at the sources of Agate creek, was, 
in many respects, similar to this outcrop; if this be regarded as the continuation of the intru- 
sion, a very sudden turn and change of its direction exists, which it is difficult to explain. 
Further observations are required to determine the exact position, direction, and form of the 
ridges, which I cannot claim to have faithfully represented on the map, it having been impos- 
sible to extend my observations far beyond the line of the survey. It may, perhaps, be found 
that there are two or more erupted dykes. 
Granite of the summit.—At the summit of the pass the hills on each side are of granite; which 
is nearly white and of a fine grain. "The mass is compact and tough, and has a sub-crystalline, 
vitrified appearance, as if it had been partly fused. Some of the surfaces of the broken frag- 
ments were observed to be drusy with minute quartz crystals. The valley or gap between the 
