A GEOLOGICAL ROMANCE. 225 



of glass fragments which have riblike edges on their flat sides 

 (Fig. 3, a). 



The nature of the force which caused the eruption may thus be 

 understood from the study of one little grain of the dust, but much 

 more extended observations are needed in order to make out the 

 place where the great convulsion took place. It will, perhaps, never 

 be known what particular volcanic vent was the source of this ash. 

 Different deposits may have come from different places. But it 

 seems possible that it all came from the same eruption. There can 

 be no doubt that the volcanic disturbances occurred to the west of the 

 Great Plains. No recent extinct volcanoes are found in any other 

 direction. This conclusion is corroborated by the fact that the dust 

 is finer in eastern localities and coarser nearer the Rocky Mountains. 

 In a bed near Golden, in Colorado, seventy-three per cent, by weight, 

 of the dust consists of particles measuring from one fourth to one 

 thirty-second of a millimetre, while some from Orleans, in Nebraska, 

 contains seventy-four per cent of particles measuring from one six- 

 teenth to one sixty-fourth of a millimetre in diameter. Still finer 

 material comes from the bluffs of the Missouri River near Omaha. 

 Evidently the coarser particles would settle first, and if the dust is 

 finer toward the east, it must be because the wind which brought it 

 blew from the west. Most likely the eruption occurred somewhere in 

 Colorado or in New Mexico. 



It may be asked how it can be known that the dust was carried 

 this long distance by the wind. May it not as well have been trans- 

 ported by water ? The answer must be, in the first place, that showers 

 of the same kind of material have been observed in connection with 

 volcanic outbursts in other parts of the world. One such shower is 

 known to have strewn the same kind of dust on the snow in Norway 

 after a volcanic eruption in Iceland, and after the great explosion 

 on Krakatoa, in 1883, such dust was carried by the wind several 

 hundred miles, and scattered over the ocean. If this ash had been 

 transported by water, it would not be found in such a pure state, but 

 it would be mixed with other sediments. There would, no doubt, 

 also be found coarser fragments of the volcanic products. On the 

 contrary, it appears uniformly fine. No particles have been found 

 which measure more than one millimetre in diameter, and less than 

 one per cent of its weight consists of particles exceeding one eighth 

 of a millimetre in diameter. In seven samples taken from different 

 places the proportions of the different sizes of the grains were about 

 as follows: 



Diameter of grains in millimetres. 

 Percentage of weight of each size 



i-i 

 0.1 



i-i 

 0.1 



1 JL _L_.-L_ --L-]- 

 S - ltT 16 3 2 3i O 



19 37 32 



1 JL 



6 4 _ 1 -1* 



9 



1.1, 



1 2*^oT> 

 1 



