NATURAL MOUNDS 



713 



one would have to suppose a circular segregation of hard material, 

 which certainly would call for unusual conditions. It is true that 

 analogous topographic forms have been produced by such conditions 

 as shown by Gilbert and Gulliver' as existing in the Tepee buttes 

 of eastern Colorado. In that case the symmetrical form is due to 

 the protective influence of a calcareous core, which is supposed to 

 have been of fossil origin. In the case of the mounds, however, no 



Fig. 2. -Section of a mound 3 miles northwest of Dardanelle, Ark., showing 

 that the material composing the mound is of different color and texture from the under- 

 lying subsoil. 



one has reported any protective material, except in a few cases where 

 gravel has been noted in more abundance in the mounds than else- 

 where. Gravel might answer as a protective cap, but such accumu- 

 lations are not universal, for in hundreds of cases observed by the 

 writer in Arkansas no gravel is to be found, either in the mounds or 

 in the soil in the vicinity. 



I G. K. Gilbert and F. P. Gulliver, "Tepee Buttes," Geological Society of Amer- 

 ica, Bulletin, Vol. VI pp. 333-42, Plate 17. j 



