570 Hillocks or Mound-Formations of San Diego, Cal. [September, 
later stages of the formations large excavations are sometimes 
made by the burrowing of animals, which are afterwards filled 
with débris, while the matter thus brought to the surface remains 
to augment the elevation. Hills formed in open spaces by ani- 
mals do not constitute nuclei for mound-formation ; composed as 
they are of a substratum in which no grass or other vegetable 
takes root and protects them from dissolution, they crumble away 
leaving but a bare and level spot. 
To recapitulate; in the incipiency of the formation the eleva- 
The dotted lines represent the orig- 
Fic, 3.—Ideal profiles of successive stages of the formations. 
nal surface of the earth. 
tion is composed entirely of a deposit heaped often abruptly » 
about the plant (Fig. 2, Fig. 3 2), but pretty soon the influence of oe 
erosion is manifest in the subsidence of the base. T 
