DEPOSITION OF MARL-LOESS 167 



subject to fluctuations of level amounting to many feet. In stages of 

 low water it is supposed that the terrestrial species of mollusks took pos- 

 session of the shores to the water's edge, and on the ensuing rise are 

 believed to have been floated or washed into the body of water and in- 

 corporated in its sediments. 



The waters are thought to have been in general surcharged with silt. 

 It is a well known fact that molluscan species rarely inhabit waters 

 highly charged with such material, the Missouri and other rivers of a 

 similar character being nearly destitute of molluscan life in their muddier 

 portions. It is believed that it is to the presence of the high silt con- 

 stituent, taken in connection, perhaps, with the low temperature of the 

 waters, that the general absence of aquatic species is due. 



In the interpretation of the aquatic species found near Mounts (see 

 page 162), several explanations present themselves : (1) The deposit may 

 have accumulated through the settling of wind-transported dust into a 

 local pool subsequent to the disappearance of the general body of water 

 from the region ; (2) the deposit may have been formed in the general 

 body of water during a stage when because of deficiency of supply the 

 whole body was I'elatively free from silt ; or (3) the deposit may have 

 accumulated in a bay of the general body of water, in which, from its 

 more or less detached situation, currents circulated with less freedom 

 and were less highly charged with silt. As to the first supposition, it 

 may be said that the topography, the color, texture, and composition, and 

 especially the richness in calcareous constituents, all point to the deposit 

 as being a part of the marl-loess rather than the loess of the common 

 type. The second supposition can hardly be true, as the conditions 

 favoring aquatic life, if at all general, would almost certainly have been 

 marked by aquatic species at some of the other numerous fossil locali- 

 ties. The third supposition, or that assuming deposition in a more or 

 less detached bay, where the waters were clearer than in the main body 

 of water, is believed, on the whole, to be the most satisfactory of the 

 three explanations proposed to account for the occurrence. 



DISTRIBUTION OF THE MARL-LOESS 



The marl-loess is much more abundant on the east side of the Wabash 

 river than on the west, though the difi*erence is not so conspicuous as 

 might appear at first sight because of the greater abundance of the com- 

 mon loess, with which the marl-loess is likely to be confused, on the 

 same side. Assuming the deposition to have taken place under fluvio- 

 lacustrine conditions, it must be postulated that the main current hugged 

 the western shore. Along the line of this current little deposition would 

 take place, while farther away, where the waters were quieter, the deposi- 

 tion would go on rapidly. 



