1879.] Hillocks or Mound-Formations of San Diego, Cal. 569 
impede the drainage of water which falls among it, and so these 
groups of plants serve to diffuse the currents—which would 
otherwise be concentrated into gulleys—whose meanderings may 
be traced in all directions among the mounds, thus conducing to 
the symmetry of their form and arrangement. 
The influence of wash in these results is the most marked on 
moderate slopes, though sometimes seen on quite steep ones and 
on comparatively level places, but if upon levels, the latter are so 
situated as to receive the gathered waters from neighboring 
slopes. Ina situation of this character near at hand the water, 
after traversing a surface of considerable extent among fully 
developed mounds, converges into a gully and a surplus flows off 
to the sea. 
Evidences of the potent agency of the winds in results of 
greater magnitude than these need not be adduced. We need 
only refer to the sand dunes of Scotland and the shores of the 
American lakes. It is a matter of common observation here 
that during the prevalence of one of the “sand storms” of a few 
hours duration, which visits us once or twice annually, several 
inches of dust is deposited in places suited for its lodgment, yet 
the work here ascribed to the wind is mainly carried on by pre- 
vailing breezes from the ocean, In situations exposed to concen- 
ý Fic. 2. aap areas with earth heaped about them as in first stage of formation. 
rom nai 
trated wind currents or their sweep over loose earth or traveled 
roads, the cones are the most sharply defined, showing that in’ 
such circumstances the work goes more rapidly on. . 
_ Asa minor and exceptional agency I may mention that in i the — 
