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.