^OLIAN DEPOSITS 



337 



from volcanoes and caught up by atmospheric currents, as de- 

 scribed on p. 122, are sometimes carried long distances to be 

 again deposited either on land or in the water, forming loose, 

 often flour-like deposits of varying thickness. At various points 

 in Colorado, Kansas, Nebraska, Montana, and other of the West- 

 ern states, are remnant beds of fine volcanic dust such as must 

 originally have covered many square miles of territory, the ma- 

 terials of which were de- 

 rived from sources now 

 wholly obscured.^ The 

 illustration given on PL 

 28 is from a photograph, 

 taken by the writer/of 

 one of these beds in the 

 lower Gallatin valley, 

 Montana. From the 

 height of the man's 

 shoulder to his feet the 

 bed is of pure glassy dust, 

 Tery light gray in color, 

 and so fine and light that 

 when thrown into the air 

 it floats away at the slight- yig. 37.— Showin^utlin^of shreds of vol 



est breath. 



Figure 



37 



canie dust, as seen under the microscope, 

 shows the appearance of 



this glass as seen under the microscope. Beds of this nature up- 

 wards of 4 feet in thickness occur underlying the loess or surface 

 soil along the Eepublican Eiver in Nebraska and Kansas and 

 even as far east as Omaha in the first-named state. The source 

 of their materials is problematical. 



These aBolian deposits are of very recent origin, and the 

 beds loosely coherent. There are, however, good reasons for 

 supposing that similar processes were carried on in the earlier 

 stages of the earth's history; but that the peculiarly susceptible 

 deposits have since undergone such extensive alteration as 

 to be no longer recognizable as wind-drifted materials. Where 

 the material still exists as a surface deposit, it undergoes 

 ready decomposition on account of its porosity and easy permea- 



1 See On Deposits of Yolcanie Dust and Sand in Soutliwestern Nebraska, 

 Ttog. U. S. National Museum, Yol. VIII, 1885, p. 99. 



23 



