SOUTHERN PACIFIC LINES 63 
which consists of caliche. This material occurs in most parts of the 
West and is frequently mistaken for stratified limestone. Much of it 
is hard and white or nearly white, and it consists largely of calcium 
carbonate with a greater or less admixture of sand. It owes its origin 
to water rising through the porous materials by capillary attraction 
and depositing calcium carbonate on evaporation at and near the 
surface. Doubtless the rate of accumulation is very slow, and ordi- 
narily the caliche or at least the thicker bodies of it are found only on 
old plains or terraces in regions of low precipitation. The thickness 
ranges from a few inches to 10 feet or more. The pit west of Seguin 
gives a fine exhibit of this material and its relations to the gravel and 
sand 
Just west of Seguin the Midway group,“ or base of the Tertiary 
system, is brought to the surface from under the Indio formation by 
the regular rise of the strata to the west. It consists of shale with thin 
layers of sandstone and clay and is sufficiently hard to constitute 
ridges in places along its outcrop zone. Such ridges attain consider- 
able prominence in the Mill Creek Hills, west of Seguin, which, how- 
ever, are gravel-capped, and in the hills that extend southwestward 
to San Anto 
The contact between the Indio and Midway formations on the 
northwestern margin of Seguin and that between the Midway forma- 
tion and the Grolaseee strata, 3 to 4 miles to the west, are covered 
by alluvium in the lowlands along the Guadalupe River. There is, 
however, a good outcrop of a fossiliferous and ferruginous sandstone 
of a horizon high in the Midway near the old ferry 214 miles above 
the Seguin power house. 
The Guadalupe River, which is crossed just east of Hilda station 
(McQueeney village), is a large and beautiful stream that drains a 
broad area of the Edwards Plateau (p. 74) and south-central Texas and 
“* The Midway consists, for the most 
part, of dark-gray joint clays with asso- 
urtle-back, or 
Midway clays of this area. 
ness is between 250 and 300 feet. In 
Guadalupe County the lower Midway 
strata crop out characteristically only 
along a slender faulted tongue south- 
west of Staples, on the San Marcos 
River. Most of the Midway surface 
material is a very heavy dark-gray clay 
which weathers into a soil indistin- 
guishable from the river silts, and the 
usual vegetation is stunted mesquite. 
ar Counties must 
have formed a synclinal basin during 
the early Eocene, for marine conditions 
Ppp 
tently, 
waters were for the most part shallow, 
for Ostrea and Cerithium are the most 
common species at the lower Indio out- 
crops. ven these, however, record 
the continuance of the general marine 
conditions of Midway time and indi- 
cate that there was a near-by retreat 
for the very considerable number of 
Midway species that persisted into 
Wilcox time. 
