DYNAMICAL GEOLOGY. 199 



cavities, channels, and tunnels in sand-dunes, clay, and loess 

 deposits, and in glacier ice. 



The formation of sand-dunes is due to the driving action of 

 prevailing winds blowing over flat sea-boards and arid inland 

 districts. Lyell, De la Beche, and Elie de Beaumont were 

 among the earlier investigators of sand-dunes, and later authors 

 have added much to our information on the changes of shape, 

 the mode of travel, and the particular kinds of sand character- 

 istic of the dunes in various localities. 



The clay deposits so widely distributed in the Pampas of 

 South America were considered by A. Bravard in 1837 to be 

 aeolian or wind-blown deposits ; but Burmeister regarded 

 them as fluviatile in origin, and Santiago Roth as partially 

 marine and partially fluviatile in origin, afterwards altered by 

 the growth of vegetation. 



The term " loess " has been applied to yellowish clay or 

 loam deposits, which were first described in the Rhine Valley, 

 and have been found to be present sometimes in remarkable 

 thickness over wide tracts of country. Baron von Richthofen 

 found in China that these deposits attained thicknesses of 

 1500 to 2000 feet, and occurred locally as high as 7000 feet 

 above sea-level. He noted the want of stratification and the 

 uniform character of loess deposits over great distances, its 

 constituents being invariably the finest particles of sand, clay, 

 and limestone, no matter what the nature of the ground might 

 be upon which the loess had gathered. He further observed 

 its porous structure, and showed that the rootlets of grass 

 growing on its surface gave origin to pipes similar to those 

 which perforated the whole mass. Another important feature 

 was the rich occurrence of remains of land molluscs, and of 

 herbivorous and other mammals, whereas fresh-water shells 

 were absent. Upon the evidence of those observations, Von 

 Richthofen concluded that the loess had originated as wind- 

 drift. And he pointed out how the dry, fine-grained material, 

 readily transported by wind, would naturally tend to accumu- 

 late on vast steppes covered with grassy vegetation. At the 

 same time, Von Richthofen recognised a "lake-loess" in 

 certain localities, in the formation of which water had 

 participated. 



This explanation 01 Von Richthofen's was then applied to 

 European occurrences of loess deposits, but the question 

 seems to be one which has to be determined independently 



