ROUNDING AND SORTING OF SAND GRAINS 715 



As to their occurrence, it may be repeated that footprints of 

 terrestrial animals are best preserved in the clays of the desert 

 playas, and the flood-plains of rivers, while those formed on the 

 seashore are commonly obliterated again by the action of the re- 

 turning tide and the waves. Tracks and burrows of worms, and 

 trails of molluscs and Crustacea, on the other hand, point more gen- 

 erally to a seashore or the borders of interior tideless seas, or, again, 

 more rarely to river flood-plains. 



12. Application of These Structures in Determining 

 Position of Strata. The importance of discriminating between 

 the original structure as here described and its reproduction in 

 reverse, in the overlying stratum, will be appreciated when it is 

 considered that strata often stand vertical, and are even overturned. 

 The determination of the upper and lower surfaces of the strata 

 may be the only means for recognizing the superposition of the 

 strata of a region. In all cases, the surface on which the structure 

 (ripple-mark, impression, etc.) was originally made is the upper 

 surface of the stratum, while the surface having a reverse repro- 

 duction of the structure (raised footprint, mold of ripple-mark, 

 etc.) is the lower surface of that stratum. 



13. Rounding and Sorting of Sand Grains and Wearing 

 OF Pebbles. This subject has already been discussed in previous 

 sections, but may be briefly summarized here. Sand grains under 

 0.1 mm. in diameter will not be rounded by wave action upon the 

 coast, for they are held in suspension, and, moreover, capillarity 

 provides the adjacent grains with a separating cushion of water 

 which prevents mutual attrition. Larger grains may under favor- 

 able conditions be rounded by the waves. They are even better 

 situated in this respect in river beds. The most efficient agent of 

 all in the rounding of grains is the wind, as it it is also the most 

 efficient sorting agent. 



Pebbles are rounded largely by the action of running water 

 and by waves. That the wind is able to do work in this direction 

 is shown by the rounded pebbles of the Hamada or stony desert, 

 where water activity is absent. It has been repeatedly asserted that 

 there is a decided difl^erence in form between pebbles of river bot- 

 toms and those of the shore. The former are said to be typically 

 flat, from the fact that they are shoved along on the bottom of 

 the stream, while the latter are rounded because they are rolled about 

 by the waves. The reverse has also been held. Until more detailed 

 observations on modern shores and river bottoms are made, no 

 such general criterion can be accepted. As a matter of fact, flat 

 pebbles abound on many shores where the material from which 



