12 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
granitic rocks, and often micaceous or gneissic pebbles, which increase 
rapidly in quantity and size toward the Colorado, and are found as far 
east as the Brazos, and even beyond it in places. 
The drift of the Colorado is composed, for the most part, of pebbles 
of flints, chert, quartz, feldspar, agate, granite, metamorphic limestone, 
and jasper, from one sixteenth of an inch to six and more inches in 
diameter; while that of the Brazos consists of flints, quartz of various 
colors, agate, jasper, cretaceous fossils, and fragments of hard Creta¬ 
ceous fossiliferous limestone, and Lydian stone. On both rivers the 
size of the pebbles grow gradually less towards the gulf. These peb¬ 
bles, outside the flint and other material derived from the Cretaceous, 
correspond to a large extent with the rocks found in the Central Min¬ 
eral Region, and are without doubt largely derived from that locality. 
Silicified wood is also abundant in the area covered by the Tertiary, 
and is derived from that formation. 
In the third district, while such siliceous pebbles are not entirely 
absent, they are very much smaller and more infrequent, forming de¬ 
posits of much less extent. The gravel is largely made up of ferru¬ 
ginous material, such as nodules of indurated sand, or carbonate of 
iron, and have their origin in the iron-capped hills which cover so 
large a portion of Eastern Texas. Silicified wood is abundant, de¬ 
rived from the adjacent Tertiary deposits. A certain quantity of the 
siliceous gravels also exist, which are doubtless derived from the old 
mountain range which crosses the Indian Territory into Arkansas, and 
it is possible that some of the drift of the Mississippi has also been 
diverted into this region. 
North of the Central Mineral Region and west of the Cretaceous 
and Carboniferous contact, the drift is somewhat different, except on 
its southern border, where it is principally derived from the materials 
found in the Llano region. The gravel is largely composed of flints 
from the Cretaceous and fragments from the surrounding rocks of the 
Carboniferous, together with bright colored pebbles, often cemented 
together, which are undoubtedly derived from the Triassic of the 
Dockum beds. It origin will be found in the Wichita Mountains, the 
mountains of New Mexico, and possibly Trans-Pecos Texas, which 
were the enclosing hills of the waters under which they were deposited. 
One character of pebble is very widel}'- spread, and I have not yet 
found in situ the material from which it was derived. It is a quartzite 
of a brown color, frequently of a decidedly purplish cast, which I have 
observed almost everywhere west of the Brazos. I have not yet seen 
it east of that stream. I find it in the Colorado drift, in the Coal Meas¬ 
ures and Permian area, on top of Double Mountains, in the basaltic 
hills of Fort Inge, in the Rio Grande basin, and in the mountains of 
Trans-Pecos Texas. Its origin will probable be found somewhere in 
the mountains of New Mexico. 
