. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. ^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.^ T
. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. ^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. Please note that these images are
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectpetrolo, bookyear1913
