The practical value of the service of the geological profession is, with
every year, being more and more appreciated, especially among people who
are developing the mineral resources of our country. Nevertheless, we
still hear men who speak of geologists as theorists that render our
profitable industries but little assistance. It is true that much of the
work that geologists do has but a remote bearing on practical questions.
The fact is that in geology, as in other sciences, one can never know
when a purely scientific observation may turn out to have a practical
application. Paleontologists who study the minutest details of fossils
have been held up as impractical people, even though their science has
more than once proved to be of the greatest practical importance for the
finding of valuable natural deposits. Certainly those who have been most
prominent in the promotion of paleontology as a science have seldom, if
ever, had any economic motive in the pursuit of their work. I think the
same is true of our leading petrographers. I believe that the men who
have advanced the science of geology most, have seldom contributed much
to the practical application of the principles they have discovered.
Much scientific work naturally appears unprofitable or useless to the
uninitiated. I shall here relate a case that suggests how entirely wrong
it may be to regard as of no economic value any geologic fact, however
insignificant it may appear.
In the summer of 1890 I took occasion to make a trip to the Black Hills
in South Dakota in order to profit, as I could, by a few weeks' tramping
in this interesting region. Going one day in a southwest direction from
Minnekahta, to look for fossil cycads, I stumbled on a block of
sandstone with a rather smooth surface on which were some peculiar
markings, such as I had never seen figured or described. The rock was
evidently a block from the Dakota sandstone. Its smooth upper surface,
which represented a bedding plane, was covered with a thin coating of
silt or fine clay which adhered to the block. The markings were in this
clay. They were straight, shallow grooves from one-half to two inches in
length, and from one-sixteenth to one-eighth inch in width. They were
joined into patterns in which some sprang out from the sides of others
and again themselves sent out other branches. Some crossed each other. I
noticed that there was a quite uniform angle of divergence in these
branches, and I was able to make out that this usual angle was about
sixty degrees. I also noted that the grooves narrowed to sharp points.
Somehow, immediately I concluded that the cracks were the result of ice
crystals, and I at once saw the propriety of frozen water having existed
in an age during which deciduous trees began to appear. This was theory.
We have since that time learned to know that cold climates far antedate
the coming of the dicotyledons.
As I had no suitable photographic equipment, I took pains to make
accurate drawings of a part of the pattern as it appeared on the rock.
My original drawing is shown in Plate I. A brief description of these
markings was later furnished in the Scientific American, of February 19,
1895.
It took me some years to find any similar markings again. In the early
spring of 1903 I had occasion to make a visit to Mexico, when I spent a
half day in Ojinaga, which is a little village south of Presidio, in
Texas, on the Mexican side of the river. Some sidewalks in this little
village are built of flags of limestone belonging to the Eagle Ford
formation. To my great delight I found some of these slabs having
precisely the same kind of markings that I had noted on the sandstone in
Dakota. Naturally I attached some importance to the fact that the Eagle
Ford corresponds quite closely in age to that of the Dakota sandstone.
Both were made at about the beginning of the upper Cretaceous age. I
noticed here a considerable variation in the closeness of the patterns
of the markings. Occasionally they were found as separate single lines,
several inches removed from each other; and on other rocks they would
be found crossing in close networks. In the summer of 1904 I again found
my ice markings on a layer of arenaceous limestone in the same formation
in the Big Bend country in Texas. This time I collected some specimens
which were subsequently photographed. One of these photographs is shown
in Plate II. Again in 1906 I noticed the same markings on some thin
sandy flags which occur in the Del Rio clay near the city of Del Rio. In
this case the needle-like crystals were somewhat more slender than those
previously seen, and some were slightly curved and somewhat more
elongated. These of course interested me as showing the occurrence of
freezing temperatures no doubt at a somewhat earlier time than that
pertaining to my previous observations.
During all these years my residence was in Illinois, and I was naturally
watching for similar markings in recent mud, resulting from late and
early frosts. I found them in the fall of 1909. At this time some
excavations were being made in the loess in Rock Island, when some rains
fell in the late fall. These rains evidently happened to give the mud
the amount of moisture necessary for such crystals to develop, as the
ground froze. The rains had washed the loess extensively, and I found a
number of places where it lay redistributed, with a fairly smooth
surface. It was evident that the moisture content of the ground,
together with the temperature conditions, determined the size and the
closeness of the frozen patterns. In places the crystals were long and
slender, in others they were short and stout. At some points they were
straight and in others slightly curved. Here and there the patterns were
close enough to resemble the fine lines which we sometimes notice in the
hoarfrost on windowpanes. In other places the crystals occurred in
radiating groups, and elsewhere they would form scattered separate
units. For preserving a record of what I saw, I poured plaster over
several patterns and had these casts photographed, as appears in Plates
VIII, IX, X. Placing these side by side with the photographs of the
patterns I have photographed from the Eagle Ford, it appears to me that
no doubt can be left as to the origin of the markings found in the
fossil state.
Recently I have found that these ice crystal marks are quite common at
one horizon in the Eagle Ford beds of Brewster County, in Texas. There
is also a layer in which they can be usually seen in the 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 so arranged in the mine that these
horizons can be entered on both sides of a fault fissure. The problem of
locating the depth of the desirable horizons in the mine in question
would be easy enough, if it were not for the fact that the outcropping
rocks consist of a series of sediments with few characteristic fossils.
Most of the fossils which occur extend through a range of several
hundred feet and the beds themselves are quite uniform in character,
consisting of alternating thin layers of impure limestones and marls. An
attempt was made to correlate the outcropping beds by close examination
of the layers exposed, but the result was not very satisfactory. A close
scrutiny made of each layer on the section resulted, however, in the
finding of two features that enabled me to measure the throw of the
fault under investigation. Interbedded in the Boquillas flags there are
some thin layers of bentonite, which are quite persistent and can be
followed for several miles. By comparing the distances between these
layers and by taking note of their individual thickness, it was possible
to make a correlation that seemed to be correct. But the proof sought
fell just short of being certain. In cases of this kind one always looks
for corroborating facts to check one's conclusions. I found this check
in the discovery of the layer which carries ice crystal markings in
these beds. The layer had a definite relation to the seams of bentonite,
and, with this additional evidence, I was confident there was no
possible chance of a mistake. It enabled me to locate not only the right
horizon but also a horizon in the underlying heavy Comanchean limestone,
which is water-bearing, and which must be avoided to prevent serious
injury to the underground operations. I need not add that the
information obtained was of real practical value in this case.