E. T. DUMBLE—SOURCES OF THE TEXAS DRIFT. 
11 
SOURCES OF THE TEXAS DRIFT. 
By E. T. Dumble, State Geologist. 
Read March 5th , 1892. 
This paper is designed to indicate, only in the most general way, the 
sources from which some of the drift materials have been derived, 
which are found so widely scattered over Texas. 
For this purpose, the area may be divided into four districts: First, 
Trans-Pecos Texas, the valley of the Rio Grande, and the country west 
of the Nueces—Rio Grande divide ; second, the country between the 
Nueces and the Brazos ; third, that between the Brazos and the Sabine ; 
and fourth, the area west and north of the Cretaceous and Carboniferous 
contact, or Northwest Texas according to the Survey division. These 
are not intended to be understood as exact lines of division, because 
no exact delineation can be made, as the materials of the several dis¬ 
tricts overlap and are intermingled in many places and in many ways. 
The drift material of the Rio Grande, as observed in the bluffs and 
capping its banks, is composed of pebbles of flint, quartz, chalcedony, 
agate, obsidian, jasper, pitchstone, flints, and fragments of limestone. 
Agates, both the mossy variety and those of beautifully banded struc¬ 
ture, are found abundantly; porphyry and quartz also occur and, after 
reaching the Tertiary, silicified wood makes its appearance in greater and 
greater proportion. Continuing down the river, the pebbles, although 
of similar character to those above, become gradually much smaller. 
Throughout the whole length of the river, these gravel deposits, which 
vary in depth from one or two feet up to twenty and thirty feet, are 
more or less indurated and mixed with sand, and more commonly with a 
tufaceous limestone, thus giving the true character of the Reynosa beds. 
The origin of this drift is readily traceable to the mountainous region 
of Trans-Pecos Texas, in which nearly every variety of pebble found 
here can be observed in its original location and forming the local drift 
as well. The flint and limestones are, of course, derived from the Cre¬ 
taceous material bordering the river. 
After crossing the divide between the Nueces and the Rio Grande, 
we come into a different character of drift. It is almost entirely made 
up of pebbles of flint and limestone, which are mixed with more or 
less sand, but frequently exhibiting the Reynosa phase. The quantity of 
pebbles of igneous and metamorphic materials, found along the Rio 
Grande, is almost entirely wanting in this region, and we have instead 
that which has been derived from the erosion of the Cretaceous rocks 
of the Balcones region. As we travel eastward, we begin to find mixed 
though this flint and limestone pebble, fragments of quartz, feldspar, 
