684 J. A. UDDEX COMPOSITION OF CLASTIC SEDIMENTS 



roadbeds by riinning railroad trains. The material which is lifted high 

 enough to come in through the windows and doors of passenger coaches 

 is mainly dust. Among 13 samples collected in coaches in different 

 parts of the United States only one had as much as 17 per cent of fine 

 sand, and one had less than 1 per cent. In five of these samples the 

 maximum occurs in the grade of very fine sand, which is next in fineness 

 to the maximum grade of the dune sand; in seven of the samples it 

 occurs in the coarse dust, and in one it is in the next finer grade. The 

 small percentages of the coarser grains is no doubt in part due to the 

 reduced velocities of the currents entering the coaches. Analogous 

 causes may have affected the perfection of the sorting in these samples, 

 which varies considerably, ninety parts in a hundred being distributed 

 among four grades in some instances and between only two in some; 

 but the differences in the speed of the trains and the di-fferences in the 

 mechanical composition of the surface deposits along the railroad must 

 also be taken into account. Xor was the sampling uniform. In some in- 

 stances the dust was taken after heavy winds and in others during calm 

 Aveather ; in some cases it was gathered up from the window-sills, and in 

 other cases from the seats in the coaches. Some of it was brushed from 

 the wearing apparel of a passenger. Taking all these modifying circum- 

 stances into due consideration and remembering that the currents of 

 wind which follow a running railroad train are quite as powerful as the 

 currents next to the ground in the heaviest wind storm, the composition 

 of this dust may be taken to indicate that fine sand is too heavy to be 

 effectively kept from settling in such winds; that very fine sand and 

 coarse dust are just on the limits of the size which is subject to effective 

 suspension. 



Similar inferences are made from the composition of some dust gath- 

 ered on a window-sill 3 feet above the ground in a building at Yuma, in 

 Arizona. 



Volcanic dust forms another class of atmospheric sediments which 

 are transported under unusually favorable conditions. It is launched 

 from great heights, to which it never could have been raised by the con- 

 vection currents of the lower part of the atmosphere, and it is carried 

 by the upper currents, where transportation is much more swift than 

 below. Nearest the volcanic outburst there is no maximum limit to the 

 size of volcanic fragments which may fall, but beyond the distance of 

 the influence of the* projectile force, which seldom perhaps exceeds a 

 dozen miles, their size is determined by the sorting action of atmos- 

 pheric currents and hence will be a true exponent of the nature of this 

 action. A sample of volcanic dust which fell on the coast of Norway, 



