228 POPULAR SCIENCE MONTHLY. 



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 

 * — ► tf 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). 



1 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 



Fig. 7.— Changes in . . » , , 



the Wind as re- winter. I he most conspicuous ot these marks are 

 corded by the some triangular and V-shaped molds of the stems 



Ripple Marks. ,, r ^ o-t iij £ 7 



and leaves 01 sedges, Siliceous skeletons of chara 

 and filamentous algse 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 vol- 

 canic 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 ima- 

 gine, if you can, a whole State, clad in the verdure of summer, sud- 

 denly, 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 be- 

 ginning 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 de- 

 scribed, 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 



