846 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. tnuLL. 213. 
High Island, Big Hill, and Damon Mound in Texas, which rise from 
40 to 80 feet above the surrounding plain and contain several thou- 
sand acres. At the other end of the series are the low, barely percep- 
tible swells, such as Sulphur in Louisiana and Spindletop and Sour 
Lake in Texas. Experience has shown that the latter afford the more 
favorable conditions for oil accumulation. 
STRATIGRAPHY. 
The formations which underlie the Coastal Plain belong to the 
latest geologic periods, the Tertiary and Quaternary, and consist 
largely of unconsolidated clay, sand, and gravel. Some of the sand 
beds have become cemented, forming sandstones, and there are occa- 
sional beds of limestone, but these are relatively inconspicuous. The 
region has been repeatedly elevated and depressed and the coast line 
has migrated back and forth across it many times. 
The formations of the Coastal Plain are briefly described below: 
Beaumont clays. — These are brown, blue, and yellow clays contain- 
ing nodules of limestone, also brown and bine sands and cypress logs; 
they have a thickness of 25 to 400 feet; they generally form clay soil 
and underlie the coastal marsh and prairie bell. 
Columbia sands. — These are white, yellow, gray, and mottled sands 
with beds of blue and yellow clay and a heavy bed of graved at base. 
They occupy a broad belt inland from the Beaumont clays and pass 
under the latter toward the Gulf. They have a thickness of 50 to 200 
feet and form sandy and gravelly soil. 
Lafayette sands. — These are blue and red thinly laminated clays 
and red and brown cross-bedded sands and gravels. They have a 
thickness of from 30 to 375 feet, form sandy soil, and are discrim- 
inated with difficulty from the Columbia. 
'Buried, beds. — These beds do not outcrop at the surface, being 
entirely concealed by overlapping later formations, and are revealed 
only by drilling. They consist of (a) 300 to 480 feet of blue, brown, 
and gray clays and sand with thin beds of limestone and containing 
small quantities of oil ; (b) 200 feet of blue clays and thin-bedded, 
irregularly deposited sandstones; and (c) 300 feet of blue, red, and 
gray clays and sands; thin-bedded limestones; limestones dolomitized 
and associated with sulphur, gas, and petroleum, and the Spindletop 
oil rock. 
Frio clays. — These consist of 2G0 feet of variously colored thinly 
laminated clays, containing gypsum crystals and calcareous con- 
cretions. 
Su mtnary. — The foregoing descriptions of the Coastal Plain forma- 
tions are necessarily very much generalized since the formations 
themselves vary greatly from place to place. The logs of closely 
adjacent wells present only a general resemblance, and it is impos- 
sible to identify with certainty any particular bed in wells separated 
