566 Hillocks or Mound-Formations of San Diego, Cal. [September, 
and structure in accordance with local conditions concerned in 
their production, exist in many parts of California and on the 
coast north of it, and are especially abundant and well defined in 
Southern California. 
The following conclusions are based upon observations of them 
chiefly in the vicinity of San Diego: 
In their most common type the mounds may be described as 
rounded eminences, or knolls, rising from one to four feet above 
the surrounding surface or the depressions between them, and 
ranging from ten to fifty feet in diameter. They are generally 
nearly circular and distinct, but are, in some instances, confluent 
or elongated. They are separated by wide and irregular areas or 
by narrow intervening depressions, the latter containing, in stony 
places, accumulations of cobblestones. They are confined to no 
geological structure or quality of soil, and are found on sloping 
lands, on the higher mesas and lower levels. 
Any attempt at an explanation of their origin and the mode of - 
their formation must be based upon the assumption that they are 
modern modifications of the earth’s surface and are due to natural 
agencies; and evidences abound on every hand that the causes 
concerned in their production are still active in the formation of 
new and in the maintenance of the old ones; and hence in this 
vicinity they may be seen in all the stages of their growth, from 
small rudimentary cones to the fully developed knolls. 
Several: agencies acting successively or simultaneously have 
been concerned in these formations. ch mound marks a spot 
where formerly grew a shrub or cluster of shrubbery, which 
served to fix its location and which exercised an important influ- 
ence in the successive stages of its development. The shrubs 
which seem to have been chiefly instrumental in these results are 
the Rhus laurina, the Simmondsia californica and the /someris 
arborea; the former undoubtedly having been principally instru- 
mental in the creation of the more recent as well, perhaps, as the 
most ancient ones in this vicinity. These plants are fitted for the 
office they perform by the nature of their growth, which is in 
compact groups or clusters, with many stems starting from the 
earth near together, the branches and foliage forming a dense 
mass resting closely upon the ground, and with beds of massive 
roots; while the distribution of the groups is strikingly similar 
_ to that of the mounds in their typical form and arrangement. 
