VOLCANIC ROCKS—WILLIAMSON’S PASS. 67 
having taken its elevation approximately by the aneroid barometer. We were obliged to spend 
the night there without water or grass for the mules. The party during the day had met Lieu- 
tenant Parke and Mr. Preuss on their way to the Mojave camp, after having finished a recon- 
noissance in the vicinity of Tah-ee-chay-pah pass. 
Erupted rocks and agates, October 24.—The mistake that we had made, gave me an opportu- 
nity to examine the rocks in our vicinity the next morning ; they were found to be principally 
dark colored, erupted, volcanic rocks, containing an immense quantity of the most beautiful 
agates in pear-shaped masses, and in mammillary coatings upon the fissures and cavities of the 
rock, The dry beds of the little water-courses were also strewn with the agates that had become 
detached by the decomposition of the rock. Most of these specimens were milk-white, and were 
delicately shaded with parallel lines; many were hollow, and the inner surfaces were lined with 
quartz crystals. These cavities were generally at one side of the specimen; the agate being 
thickest on the side that was lowest at the time of its formation. ‘I'he mammillary crusts and . 
coatings along the fissures were of various colors—red, brown, and greenish. Their surfaces 
were smooth, and possessed a high natural polish. 
Where the small rounded masses of agate had fallen ont of the rock, they left numerous 
cavities, that give it a vesicular appearance. A peculiar green crust was very common on some 
parts of the rock, and at a little distance it looked like carbonate of copper; but it does not con- 
tain a trace of that metal. 
The position of the locality, with respect to the pass we were seeking, will be seen on the 
map. I have marked the name Agate creek opposite the stream, as the abundance of this 
mineral in its bed was its distinguishing characteristic. 
We retraced our steps, and, after gaining the broad slope of the basin, travelled to the Cow 
camp, where there-was plenty of grass and water for the refreshment of the fatigued animals. 
Having found the entrance to the pass, we returned to it the next day and commenced the 
survey. 
WILLIAMSON’S PASS. 
The position of this pass, and its direction, will be seen upon the general map and on the 
geological map of the Tejon and vicinity, Map II, Chapter XV. The pass extends from the 
Great Basin to the Rancho of San Francisquito, which is in one of the valleys of the Pacific 
‘slope. Its entrance on the Great Basin is near the meridian of 118° and latitude 34° 30’, and 
its direction a little south of west; the distance through the mountains from the Great Basin 
side to the open valley of the Santa Clara river is about 20 miles. The altitude of the summit 
level is 3,164 feet above the sea." 
In the description of the sandstone bordering the lower portions of the Pass of San Franeis- 
quito, it is stated that along valley extends in a nearly northeast direction towards the Great 
Basin, between the ridge of talcose or chloritic slates and the granite. In travelling at the base of 
the mountains on the Great Basin slope, between the Pass of San Francisquito and the entrance 
to Williamson's Pass, outerops of slate of a similar character were observed along the trail, 
forming hills on the right, at a point about half-way between the two passes. By inspection 
of the map, it will be seen that this point is in the line of the northeastern prolongation of the 
outcrop in the Pass of San Francisquito ; and I have therefore concluded that the range is 
continuous between the two points. This is also the point at which the long valley occupied 
1 See the tables in Lieutenant Williamson's Report. 
