set in squarely from the west, for the ripples are turned north and south, with the steeper slopes to the east. This direction seems to have prevailed as long as the dust kept on falling. It appears to me that these successive changes are best explained as attendant upon the passage of a cyclone, or of what our daily weather maps call a "low area." Going by from west to east, on the north, it would at first cause an east wind. This would then gradually be turned to the south and then to the west. One such rotation 
Fig. 7.—Changes in the Wind as recorded by the Ripple Marks. of the wind generally lasts a day or two. The shower must then have kept on at least for the same length of time, if not longer (Fig. 7).
There is reason to believe that this catastrophe a occurred in summer. No crayfish would be out making tracks during the cold months, and the fossil vegetation could hardly have left such plain marks if it had been buried by the dust during the winter. The most conspicuous of these marks are some triangular and V-shaped molds of the stems and leaves 01 sedges, Siliceous skeletons of chara and filamentous algæ were also found upon a close examination in some of these molds.
It is really difficult to appreciate the change such a shower must have produced in the appearance of the landscape, and the effect it must have had on animal and plant life. So far away from the volcanic source, the wind can not have laid down a layer of this dust several feet in thickness without scattering it far and wide all around. It must have covered tens of thousands of square miles. Just imagine, if you can, a whole State, clad in the verdure of summer, suddenly, in two or three days, covered over by a blanket of white volcanic ash! Many species of plants must have found it impossible to grow in such a soil. And what disaster it must have caused in the animal world! Grazing herds had their sustenance buried from their sight, and could save their lives only by traveling long distances in this loose dust. Many a creature must have had its lungs or its gills clogged with the glassy flakes floating in the water and in the air. The sudden disappearance of several mammal species near the beginning of the Quaternary age has been noted by paleontologists. Does it seem unlikely that an event like this, especially if repeated, may have hastened the extermination of some species of land animals? That many individuals must have perished there can be no doubt. Not very far away from that outcrop of the dust which I have described, one of the early settlers in this part of the State once made a deep well that penetrated the ash. Above the deposit, and some sixty feet below the surface of the prairie, he found what he described as
