o 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.

PLATE I

Plate I. Forms of frost cracks seen on the exposed flat bedding plane of
a block of Dakota sandstone in a ravine a few miles southwest of
Minnekahta, South Dakota. As sketched in the field. Natural size.

[Illustration: PLATE I]

PLATE II

Plate II. Photographs of fossil imprints of ice crystals on flags of the
Eagle Ford. The upper rock shown in the Plate is from the south side of
Cuesta Blanca in Brewster County, Texas, and shows casts of crystals
which represent fillings of sandy mud projecting slightly down into an
underlying bed of more argillaceous material. The lower