410 THE ICE AGE IN NORTH AMERICA. 



Verds, which was estimated to be sixteen hundred miles 

 broad. A similar shower which passed over the southern 

 part of France in 1846 is estimated by Ehrenberg to have 

 deposited nearly a million pounds of dust. On examination 

 he found that about one eighth of it consisted of microscopic 

 organisms, many of which belonged in South America, thus 

 showing that the wind had blown the dust across the Atlan- 

 tic. In 1835 volcanic dust was blown from Guatemala to 

 Jamaica, a distance of eight hundred miles. 



The main difficulty of applying the wind theory to account 

 for the loess of the Mississippi Valley and of Europe lies in 

 its definite relation to water-levels. For example, in the 

 Ehine Valley the loess rests in places upon elevations of 

 eight hundred feet above the river, but does not occur at 

 higher levels. This would clearly indicate that it is a water- 

 deposit. The problem has been to conceive how the water 

 could be maintained at that level in the valley of the Rhine ; 

 and the theory has been put forth, and ably defended by 

 Greikie and others, that the Rhine and other rivers emptying 

 into the North Sea were obstructed at their mouths by the 

 Scandinavian glaciers, and so the water was ponded back 

 toward the Alps to the level at which the loess ceases. 



The absence of fossils of aqueous origin in the loess may 

 be accounted for in two ways : 1. If the deposit took place, 

 as was supposed, in temporary glacial lakes, there may have 

 been no species existing in them to have left their remains. 

 This would be likely to be the case for three reasons : (1) 

 the lakes were of only temporary continuance ; (2) the tem- 

 perature of the water must have been abnormally low ; and 

 (3) the superabundance of silt might interfere with animal 

 life. 2. Professor Hilgard has suggested that the porosity 

 of the loess, which favors the continual presence and percola- 

 tion of water charged with acids throughout its mass, renders 

 it almost certain that any inclosed fossils would have been 

 rapidly dissolved. As to why this oxidizing and dissolving 

 process should have selected fossils of aqueous origin, and 

 made an exception in favor of those of terrestrial animals, 



