690 J. A. UDDEN COMPOSITION OF CLASTIC SEDIMENTS 



material which had accTimulated in a long crevice in the ice, across 

 iVrhich the wind had been drifting after the deposit had settled. The 

 dust taken near the center of the channel of the Mississippi Eiver has 

 less of the coarse admixtures, as does also that taken on snow. Over 

 level areas, as out on the ice of the river away from the banks and on an 

 open field, a greater proportion of fine dust is noticeable, due most likely 

 to a more even progression of the atmosphere permitting more of its load 

 to settle, while near places where timber or the relief of the land had 

 set up convection currents less of the finer dust seems to have been able 

 to come down. In the averages from each of these three showers the 

 medium and the fine dust are present in nearly equal proportions and 

 constitute from 74 to 87 per cent of the whole. In the samples collected 

 on the ice there is nearly three times as much of the two grades of sand 

 as in those taken on snow, owing, it seems, to the greater quantity of 

 local drift raised by the wind from bare ground along the banks of the 

 river. 



A sample of dust was taken just west of Chicago soon after the shower 

 which occurred in the latter part of February, in 1896. It contains a 

 considerable admixture of local coarse fragments, but aside from this 

 it is slightly finer than the average deposit from the same storm at Eoek 

 Island. Still another sample was collected at Maysville, in New York, 

 after this storm. This also contains a small quantity of sand, but it is 

 otherwise the finest of all the samples of the shower dust examined, hav- 

 ing a larger percentage than the rest of all the grades containing parti- 

 cles less than one thirt3^-second of a millimeter in diameter. 



List of Samples in Table 29 



320. Shower dust from Kansas City, Missouri. 



321. Shower dust from Alta, Iowa, June 7, 1895. 



322. Shower dust from Alta, Iowa, June 8, 1895. 



323. Shower dust on the ice of a small pond. Rock Island, Illinois, November, 



1894. 



324. Shower dust on the ice of the Mississippi, near its bank. Rock Island, 



Illinois, November, 1894. 



325. Shower dust on the ice near center of channel of the Mississippi, Rock 



Island, Illinois, November, 1894. 



326. Shower dust on the ice of the Mississippi, near bank, Rock Island, Illi- 



nois, January, 1895. 



327. Shower dust on the ice of the Mississippi, near bank. Rock Island, Illi- 



nois, January, 1895. 



328. Shower dust from a crack in the ice of the Mississippi, near channel, 



Rock Island, Illinois, January, 1895. 



329. Shower dust on the ice of the Mississippi, near center of channel, Rock 



Island, Illinois, January, 1895. 



