[Ill] Rocks of Presidio and Brewster Counties. 
5 
the weathering of the underlying materials, but beyond that point very 
little material of this character was observed. 
The hills along the route to Ajamitos appear to be composed entirely 
of the tuffs and accompanying lavas, and in none of the exposures seen 
at San Jacinto Peak did we find anything else. 
Similar materials were observed at Water Gate Pass and forming the 
escarpment on the western side of Green Valley. Here we found beds 
of grit overlain by conglomerate and this by a rhyolitic bed, followed 
by later flows of lava of various colors, the whole rising to a height of 
more than 400 feet above the valley. These beds are also well developed 
on Fresno creek, where they overlie the Cretaceous, and the provisional 
name Fresno beds is given them for descriptive purposes until such time 
as they may be more definitely placed. 
In the vicinity of the ranch house at Alamo de Caesario the dark- 
colored (andesitic?) lavas are cut by porphyries. At the water hole 
one-half mile east of the ranch house erosion has exposed the base of 
one of the lava flows and the markings show plainly that the direction 
of its flow was from northeast to southwest. 
Still further down the creek the grit which has been- mentioned as 
the base of the Green Valley escarpment was found to be underlain by 
a highly crystalline limestone and shales, the whole forming a series of 
about 100 feet in thickness. The beds are not entirely regular, as the . 
limestone occurs at times interbedded in the shale, while at others a 
grit or conglomerate may underlie or overlie it. One exposure showed 
this complex overlying a great volcanic breccia in which the boulders 
of lava are as much as 8 to 10 feet in diameter. Three miles east of 
Alamo de Caesario this breccia, which has a thickness of 100 feet, over- 
lies a body of obsidian, which in turn rests directly upon the lime flags 
of the Eagle Ford shale. 
The beds lying east of the Alamo de Caesario therefore appear to be 
the oldest of the Tertiary, and those to the northwest would be later. 
The only means, however, of differentiation would be in the composition 
of some of the materials and their connection with the different struc- 
tural lines which prevail. 
This series of Tertiary deposits is more nearly allied to those of the 
western portion of the United States and of Mexico than to those on 
the east. The same character of volcanic complexes occur here, though 
less in extent, that are found in Arizona and further north and west, 
and in Sonora and other parts of Mexico ; deposits which are known east 
of the Trans-Pecos, for the most part, simply as thin beds of volcanic 
ash interbedded with certain materials of the later Cretaceous and others 
of the Tertiary period, the age of which is not later than Lower Clai- 
borne, and in the Oakville beds, which are certainly Miocene. 
These deposits as a whole greatly resemble those observed by the writer 
