34 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
SOILS OF THE TERTIARY PLAIN. 
To the Tertiary or Lignitic belt, which succeeds the Coast Prairies in 
the interior, we are indebted for our great forest region, for vast depos¬ 
its of iron ore and brown coal, and for large areas of very fertile soil, as 
well as for fertilizers adapted for the preservation and enhancement of 
their fertility. 
While the soils of this great plain, derived from its numerous beds of 
sands and clays, may be classed in a general way as loams, they vary 
greatly, and are readily separable into several smaller belts correspond¬ 
ing to the geological divisions, and may well be designated by the names 
of their geological ancestry. These are, in the order of their occurrence 
from the Coast Prairies interiorward to the Black Waxy belt: The Rey- 
nosa, Oakville, Fayette, Yegua, Marine, and Lignitic. 
The Reynosa or Lafayette. —The surface of this belt is somewhat 
more rolling than that of the Coast Prairies. East of the Colorado river 
the uppermost beds of this plain consist of sand and gravel, colored more 
or less by oxide of iron derived from the iron region of the Marine belt 
north of it. West of the Colorado, the iron is replaced by lime brought 
down from the Cretaceous area lying to the north, and this forms the 
“ adobe ” rock or white limy clay of the southwest. The lower beds are 
uniformly cla}^s. 
The Reynosa formerly occupied a much greater area than at present, 
for we find patches of it well up the sides of the iron capped plateau of 
Anderson and adjoining counties, while in Southwest Texas the adobe 
and limy conglomerate extend inward, in places, to the line of the 
Southern Pacific railway, and it usually forms the tops of all the more 
important divides. By reason of this overlap this plain extends over the 
greater part of the Neocene deposits of the Gulf Tertiaries. 
In its central portion the belt comprising the Reynosa is hard to sepa¬ 
rate from that of the Coast Prairies, so closely do they resemble each 
other, but elsewhere the differences are more marked. 
The soil derived from the more sandy eastern portion is principally of 
a brown sandy character, as seen in parts of Waller, Montgomery and 
Liberty counties. 
The black waxy prairies of Washington and Fayette counties are largely 
derived from its lower or clay bed. All of these soils are fertile, and some 
of them are of the greatest richness. This is readily accounted for when 
their origin is known. Where beds of gravel occur an examination will 
show that by far the greater portion of the pebbles are granitic or feld- 
spathic rocks from the Llano region, and these are usually strongly kaolin- 
ized. The soils represent the finer sediments from the same materials, to 
which have been added those derived from all the beds north of them. 
