320 NATURAL HISTORY BULLETIN. 



favor the development of snails. The reason that snails are 

 not found fossil in the loess remote from streams is that when 

 living, they do not, to any extent, inhabit such places. 



Time. — The element of time is also to betaken into ac- 

 count. It might seem that, if loess was deposited most abun- 

 dantly where vegetation was comparatively vigorous, there 

 ought to be an abundance of plant remains in the deposit. 

 The rate of deposition, however, must have been so slow that 

 all organic matter would have disintegrated long before it 

 could have been covered and sealed in the deposit. Organic 

 remai.is can thus be preserved only when overwhelmed, es- 

 pecially in wet places, and their absence would rather mili- 

 tate against the aqueous theory. If the rate of net deposition, 

 after deducting loss by erosion, already estimated by the writ- 

 er,* namely, i mm. per year, be accepted, it is evident that a 

 stick or log, or even a leaf, would decay long before it could 

 be entombed in the deposit. At that rate a log one foot in 

 diameter, for example, would require more than three hun- 

 dred years for burial. The shells of molluscs do not similar- 

 ly disintegrate, and are preserved as fossils, partly because of 

 their composition and texture which better enable them to 

 resist exposure, and partly because all of these terrestrial 

 snails are more or less inclined to burrow, or at least conceal 

 themselves in the lowest strata of leaf-mould, etc., and their 

 shells are soon covered up.f In many respects the borders of 

 of the drift-sheets in the north, especially where morainic, 

 presented conditions similar to those now existing along the 

 larger streams. This question, however, does not concern 

 the loess of Natchez and Vicksburg, and will be discussed at 

 another time. 



*Jour. Geol., vol. VII, p. 135, and Proc. la. Acad. Sci., vol. VI, pp. 

 109 and 110. 



tTo collect sortie of our smaller species of modern snails which are 

 also represented in the loess, in autumn or during dry summers, it is 

 necessary to pull up the roots of smaller plants among which many of 

 the snails are concealed. 



