712 MARIUS R. CAMPBELL 



we may say that the choice hes among four hypotheses : animal bur- 

 rows, erosion, solution, and human construction. These hypotheses 

 involve two fundamentally different processes, construction and 

 destruction, and it seems possible to determine definitely which of 

 these processes has taken place. 



Many writers have argued that the mounds are the result of 

 erosion, but in most cases they have realized that under normal 

 conditions such forms could not have been produced, and hence they 

 have been compelled to assume unusual conditions, but rarely have 

 they stated clearly what were the unusual conditions. 



Practically all of the mounds observed by the writer are located 

 on smooth surfaces, and generally on level plains. Where the 

 mounds are close together they may touch and the slope of one 

 merge into the slope of its neighbor, forming a concave surface; 

 but where mounds are scattering, the space between is always fiat, 

 being in reaUty a part of the surface of the plain. In arguing for 

 erosion this fact has generally been neglected, or it has been assumed 

 that the inter-mound surface is not a plain. Thus Barnes,' in 

 describing the mounds at San Diego, illustrated in Fig. i, gives a 

 profile showing the mounds separated by a concave surface. The 

 mounds in this locality were examined very carefully by the writer, 

 and in every case where they were not in actual contact the space 

 between them is fiat and not concave, as represented in Barnes's 

 profile. It therefore seems that much of the misapprehension 

 regarding the origin of the mounds is due to imperfect observations 

 and assumptions that are not warranted. 



When one considers the way in which surfaces are eroded, it is 

 manifestly impossible to produce a fiat surface unless that surface 

 is at base-level, or the process of erosion is controlled by a barrier 

 or by underlying hard rocks. If the surface was at base-level, and 

 the plain a base-level plain, it would mean that the cycle of erosion 

 was practically complete, and in that event the mounds would be 

 reduced as well as other portions of the surface, unless in some 

 abnormal way the mounds were protected from the action of erosion. 

 In order to protect mounds with such symmetrical, spherical surfaces, 



I G. W. Barnes, "The Hillocks or Mound-Formations of San Diego, California, " 

 American Naturalist, Vol. XIII (September, 1879), pp. 565-71. 



