Memorial and Biographical History of Dallas County, Texas. Page: 136 of 1,110

HISTORY OF DALLAS COUNTY.

spicuous representative division of the whole
Upper Cretaceous system. This occupies the
narrow strip, as noted in the preceding topographic
description, marking the western
border of the main Black Prairie region, separating
it from the minor Black Prairie. The
outcrop of this chalk begins in the southwest
corner of the State of Arkansas and in the Indian
Territory. It crosses Red river, the
exposure continuing westward up to the
south side of the valley of that stream to the
north of Sherman, from which place it deflects
southward, passing near McKinney,
Dallas, Waxahachie, Hillsboro, Waco, Belton,
Austin, New Braunfels, San Antonio
and Spofford Junction, beyond which it
bends northward, appearing in the disturbed
mountains in the vicinity of El Paso and
New Mexico. It is distinguished above all
by its peculiar chalky substructure.
The words "limestone" and "chalk" are
used on these pages as follows: Limestone is
employed generically for species of widely
different origin and structure, namely, of five
kinds: 1. Brecciascomposed of more or less
comminuted and cemented shells of ancient
bottoms or shores. 2. Concretions or segregations
formed by the segregation of lime
in clays and sands after original deposition,
rare in our rocks. 3. Chalks are composed
of amorphous calcium carbonate, usually
more or less foraminiferous, void of laminations,
and of comparative deep-sea origin.
These may be hardened by metamorphism
into firm limestones. Hence the term
"'chalky limestones" will imply chalky origin.
4. Laminated, impure limestones, occurring

as alternating beds in sands and clays, indicative
of shallower origin then chalk. 5. Metamorphosed
limestones, or any of the above
which have undergone induration or secondary
change. All laminated limestones thus
far found in the Texas Cretaceous are in the
basal beds, and are more or less arenaceous
or argillaceous, further proving their origin
to have been in shallower water than those
in which chalk is laid down.
The rock of the Austin-Dallas chalk fornation
is a massive, nearly pure, white chalk,
usualy free from grit and easily carved with
a pocket-knife. Under the microscope it
exhibits a few calcite crystals, particles of
amorphous calcite, and innumerable shells of
foraminiferae. The air-dried indurated surfaces
are white, but the saturated subterranean
mass has a bluish white color. The rock
weathers in large conchoidal flakes, with an
earthy fracture.
In composition it varies from 85 to 94 per
cent. of calcium carbonate, the residue coisisting
of magnesia, silica and a small percentage
of ferric oxide.
The thickness of this chalk is about 500
feet. So far as observed in Texas it averages
the same thickness at Austin, Sherman and
Dallas. It is of great uniformity throughout
its extent; but there are a few local differences
in hardness, which are sometimes
due to surface induration and to igneous action,
having been converted into marble in
some places.
A great portion of the former extent of
this chalk has been destroyed by erosion, and
its western border in Central Texas is now