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Contributions to the Geology of Texas. 23 
other southern states. While on a tour to the upper Brazos, I 
discovered in the neighborhood of the town of Caldwell, strata 
of a ferruginous sandstone with numerous and well preserved 
tertiary shells. Crossing afterwards the Brazos not far from this 
town, I had a still better opportunity to see this formation along 
the steep banks of the river. It consists of alternating strata 
of brown ferruginous sandstone and of dark colored plastic clay, 
both teeming with fossils. Unfortunately, the circumstances 
poe not allow me to make a complete collection of them; the 
however, which I gathered are sufficient to prove that 
‘ees strata belong to one of the older divisions of the tertiary 
period. I have good reason to suppose that these same tertiary 
deposits have a wide range in the eastern part of Texas, though 
I am unable to give their exact limits. Tertiary fossils from Na- 
cogdoches seem to indicate that the deposits of the Brazos ex- 
tend as far as nee 
Cretaceous formation.—We come next to the cretaceous strata, 
which of all the settea formations take the most important 
part in the geological constitution of Texas and chiefly her upper 
hilly part. The immense tract of land which extends from the 
above mentioned line, connecting the Rio Grande with Red River, 
to the head waters of the Colorado and the other large rivers of 
Texas, is occupied entirely by cretaceous deposits, except a belt. 
of silurian and carboniferous strata and a mass of granitic rocks, 
oth covering comparatively a small are 
we examine first the Bas arn FEE, constitution of these 
eretaceous rocks, a striking difference from other deposits of the 
cretaceous period on the North American continent at once be- 
comes apparent; for whereas these latter, on the whole Atlantic 
coast, are almost entirely composed of loose and incoherent mate- 
rials, the cretaceous strata of Texas constitute mostly compact 
and hard rocks, some of them equalling in compactness the hard- 
est strata of more ancient secondary formations. A calcareous 
character is very commonly observed; in fact, 1 have not seen 
any sandstones or strata of clay in the whole series. Generally 
speaking, there is an alternation of compact siliceous limestones 
and less compact beds of either pure, or marly, limestone. ‘The 
former contain the silex as well diffused through their whole 
mass, as in separate concretions or nodules. The siliceous char- 
acter of these rocks, excluding the decomposing action 0 
atmosphere, almost entirely produces the general dry and barren 
aspect of the country which they occupy. Every where in the 
mountainous region, north from Austin or San Antonio de Bexar, 
on both sides of the Piedernales and San Saba Rivers, it is only in 
the valleys that a fertile stratum of soil is found; on the heights 
of the table land the bare rock appears almost every where at the 
surface, hardly supporting the scanty growth of grass and some 
scattered specimens of stunted live-oak and post-oak trees. 
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