The Dust fall of March 9, 1918. 607 



available over a wide area. The telegraphic report of 

 the Weather Bureau at 7 a. m. of March 9, 1918, showed 

 high winds prevailing throughout the southwest from 

 the Mississippi valley to the Rocky Mountains. Among 

 the higher velocities reached during the preceding night 

 were 48 miles per hour at Oklahoma City, 44 at Denver, 

 44 at Wichita. On the preceding day the storm center 

 was advancing through Utah and Colorado, and a region 

 of steep barometric gradients and strong winds passed 

 over the arid regions of Nevada, Utah, Arizona, Colorado, 

 and New Mexico, a maximum velocity of 48 miles an hour 

 being attained at Modena, in southwestern Utah. 



The velocities are sufficient to blow up into the air not' 

 only clouds of dust, but to whirl up from the ground 

 gravel of considerable size. The limit of snow cover, 

 from the Snow and Ice Bulletin of the Weather Bureau 

 of March 5, 1918, and the areas covered by rainfall during 

 the advance of the storm are shown in fig. 3. Except in 

 Colorado, and northern New Mexico, the territory sub- 

 jected to high winds was not protected in any way, aside 

 from the natural vegetal covering, against eroding winds. 

 The reports from observers and from military camps in 

 the region indicate that extraordinary duststorms pre- 

 vailed and caused much discomfort. 



The microscopic study of the dust reveals several facts 

 having an important bearing on its origin. First, it is 

 well sorted and very fine. Both of these facts indicate 

 that it has been carried a long distance in the air (accord- 

 ing to the estimates of Udden a distance which may be 

 a thousand miles or more). Next, the dust is charged 

 with abundant limonite and hematite, although kaolin is 

 not abundant and the feldspar is entirely unaltered. 

 These facts indicate that the dust is a product of physical 

 disintegration, and not of chemical decomposition, that 

 is, it is derived from a region of very arid climate and 

 not from any part of the Mississippi valley. Finally, the 

 dust is dominantly composed of feldspar and quartz with 

 very small amounts of other constituents. Therefore, it 

 is derived from a region of siliceous feldspathic rocks, 

 either granite or arkose, or a gneiss of similar composi- 

 tion. It is not derived from a region of limestone, sand- 

 stone, mica schist, or basic igneous rocks. It contains far 

 too little kaolin and its feldspar is too fresh to be derived 

 from any ordinary shale or argillite. 



