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.