Geographic Features of the Texas Region. — Hill. 73 
limestones predominate. Accompanying these greatly dis- 
turbed and metamorphosed sedimentary rocks are many out- 
crops of both plutonic and eruptive material, such as the 
peridotytes and granites of Arkansas, the granite bosses of 
Tishomingo and the red granites, basalts, porphyrytes and 
eruptives of the Witchitas. Concerning the latter no more is 
now known than was expressed as follows by Dr. Edward 
Hitchcock some years ago : 
"The rocks of these mountains are traversed by veins of 
greenstone and quartz. The latter is often porous and col- 
ored by the oxide of iron. The greenstone is the most recent 
of the unstratified rocks among my specimens, save a single 
vesicular mass, broken probably from a boulder, which has 
all the external marks of lava. It looks more like recent lava 
than any specimens I have ever met among greenstones or 
basalt. It was collected west of the great gypsum deposit, 
though in a region abounding in sandstone, and near the 
bluffs that form the border of the Llano Estacado. Dr. 
Shumard found in the bed of Red river near the same place, 
what he calls greenstone porphyry and trachyte. The spec- 
imen to which I have referred is rather augitic than trachytic. 
He says also that he found there a 'black scoria and other 
specimens of volcanic rock.' Again, on approaching the 
Witchita mountains on the return trip he describes one as a 
truncated cone with a basin-shaped depression in the summit; 
of which he seems to have judged by looking at the mountain 
from a distance. But taking all the facts into account I can 
not but feel that there is reason to presume that volcanic 
agency has been active in that region more recently than the 
trap dykes." -^ 
The structure of the system has not been studied as a whole. 
For the most part, however, it consists of almost vertical folds 
of the composing stratified rocks, as shown in the section 
along tlie Arkansas-Choctaw line," at and in the beautiful 
water gap of the Washita, along the line of the Santa Fe rail- 
road, in the Chickasaw Nation. Accompanying these are 
dykes and veins of igneous material as well as great masses of 
granite. The general trend of these folds and ridges, as seen at 
'Dr. Edward Hitchcock, pp. 146-147 of Marcy's report on Red river. 
* See Am. Jour. Sci. April, 1889. 
