vicinity of
Austin, Texas. This lies about twenty-five feet below the Austin Chalk,
near Austin. A like layer occurs about 100 feet below the Austin Chalk
in the Big Bend country. Here I have found the markings in localities
thirty miles apart. They occur at the north point of Mariscal Mountain
and in a number of places near the Fossil Knobs and on the Chisos Mining
Company property at Terlingua.

Unprofitable as observations on such a simple matter as this may seem, I
find that other geologists have given it some attention. Quite recently,
Dr. John M. Clarke[A] has figured slabs showing what has been described
as _Fucoides graphica_, by Hall. The markings figured by Professor
Clarke are undoubtedly of the same kind as those I have found in the
Eagle Ford. They occur in the Upper Devonian in New York. I also find
that the formation of ice crystals in wet mud has been observed in the
clays about Boston by Marbut and Woodworth.[B] Other observations of
similar recent markings are said to have been made by some English
geologists.

To "practical people" it may indeed appear that no more unprofitable or
more idle curiosity could be indulged in, than making observations on
what kind of crystals are formed when water freezes in mud. I must
confess that my own first observations had no motive whatever, except
for the desire to know something new; and I never expected that anything
I could learn about these fossil marks would ever turn out to have any
practical application, at least not in my own work.

But it has turned out differently. For some time, I have been called
upon occasionally to advise with regard to the finding of the ore in one
of our quicksilver mines in West Texas. It is now a well established
fact that the distribution of the ore in this mine, and I believe in the
entire Terlingua district, bears a definite relation to geological
horizons. Successful mining requires search in these horizons. The
cinnabar, as it appears, has accumulated in greatest quantity under
impervious rocks such as shales and marls along planes that separate
these from underlying rocks of more open texture, mostly limestones. The
ore has clearly come from below and has risen through fissure planes,
which in some cases separate large blocks of the Cretaceous formations.
The best ore has been found under the basal part of the Boquillas flags,
and under the Del Rio clay in the upper part of the Georgetown
limestone. The workings must be s