SCIENCE 



[Vol. XXII. No. 544 



limits of variation, and it is, perhaps, due to this fact that it is 

 so universally distributed. Tbe littoral and pelagic forms are so 

 different that they have been considered speciScally distinct. 



C. modestus is a rare form. Thus far it has been found in 

 only a single locality in Wisconsin. 



None of the American species of Diaptomus is identical with 

 those of Europe, although in some cases the relationship is very 

 close. 



D. siciUs is the common pelagic form of the Great Lakes, but 

 occurs also in smaller bodies of water. D. ashlandi has been 

 foun'l only in the Great Lakes. 



The most common species in tbe smaller lakes is D. oregon- 

 ensis. This was described by Lilljeborg from specimens col- 

 lected in Oregon, and probably is common through our northern 

 States. D. minutus is common in Newfoundland, Greenland 

 and Iceland. It occurs in some of the small lakes in northern 

 Wisconsin and in Green Late. It is likely that it occurs quite 

 generally through the northern part of North America, and pos- 

 sibly central Wisconsin is near its southern limit. 



Especial interest attaches to tbe fauna of Green Lake. This is 

 about seven miles long, with a maximum depth of nearly two 

 hundred feet. While the pelagic fauna of tbe Gx-eat Lakes is 

 quite distinct from that of the smaller lakes, we find in Green 

 Lake both sets of faunse. £>. siciUs and Limnocalamts macrurus 

 I have not found outside the Great Lakes except in Green Lake. 

 But besides these species the pelagic fauna of Green Lake in- 

 cludes C. brevispinosus and C fluviatilis, which are the charac- 

 teristic species of tlie smaller lakes. 



A more detailed account of the Wisconsin copepoda will soon 

 appear in the Ti-ansactions of the Wisconsin Academy. 



THE HILLOCK AND MOUND FORMATIONS OF SOUTH- 

 ERN CALIFORNIA. 



EY DANIEL CLEVELAND, SAN DIEGO, CALIFORNIA. 



Some time ago, in an article upon the nest of the trap-door 

 spider, which appeared in Science, I mentioned t- e low mounds 

 in which these nests in many districts are so often located, as 

 being in themselves an interesting formation. I now propose to 

 offer an explanation of the origin of the formation. 



Let me begin by saying that these mounds are not confined to 

 this vicinity, for they extend throughout this State and elsewhere 

 on this coast and in Texas; but they are more numerous and 

 better defined here than elsewhere; they are, in fact, a charac- 

 teristic of certain large areas of our territory. For this reason, 

 among others, I believe this to be the best field for observing and 

 investigating this remarkable formation. 



Lying just back of the commercial portion of the city of San 

 Diego there is a great mesa or table-land, which stretches away 

 for a distance of fi-om eight to ten miles to the valleys at the 

 base of the Coast Eange. It possesses a rich brown soil, holding 

 in many places considerable aggregations of loose stones which 

 have drifted down from the neighboring mountains and been 

 ground into pebbles. Here for miles the surface is gently undu- 

 lating, with low mounds lying as close together and as numer- 

 ous, considering their size, as the ground will permit. These 

 mounds are from one to three feet in height above their bases, 

 and are from ten to thirty feet in diameter, separated by greatly 

 varying areas which in their depressions in many places contain 

 accumulations of cobble stones. An unscientific person seeing 

 these plains for the first time might imagine that they had once 

 been densely populated by large burrowing animals which had 

 left these hillocks to mark their subterranean dwellings. 



Several theories have been advanced to account for this for- 

 mation. The most probable hypothesis is suggested by the na- 

 ture of the soil and the peculiar vegetation of these plains. The 

 soil itself is dry and hard for the six to eight months constituting 

 the rainless season. During the time of heavy rains it is soft and 

 mellow. During the time of drought it becomes almost as hard 

 as stone. 



Each mound, it is evident enough, marks the former home of a 

 shrub or. as was almost always the case, of a cluster of shrub- 

 bery, to whose agency the mound in large degree owed its exist- 

 ence. Three shrubs — Rhus laurina, Nutt. ; Simmondsia Califor- 



nia, Nutt. ; and Isomeris arborea, Nutt. — are conspicuous among 

 the large vegetation of these plains, and have been very important 

 factors in the formation of these mounds. Of these plants Rhus 

 laurina is the largest and is much more abundant than the other 

 two. It is an interesting fact that these three shrubs are con- 

 fined to this section of California, mostly to this county, and 

 that they were all first collected at San Diego about 1840, and 

 were named by the eccentric naturalist Thomas Nultall. He es- 

 tablished the genera Simmondsia and Isomeris The habits of 

 these plants peculiarly Ht them for their office of mound build- 

 ers. They grow in small compact gi-oups. Many stems rise 

 from the roots, which are large and spreading. The foliage of 

 Rhus and Simmondsia especially is dense and falls close to the 

 ground. 



Dust blown by tbe steady trade winds of the dry season is ar- 

 rested by the shrub and accumulates with the fallen leaves at 

 its base, making a steady accretion of material. In this way a 

 mound gradually rises about the plant, in time covering the 

 lower branches and in the case of the smaller shrubs — Simmond- 

 sia and Isomeris — nearly or quite enveloping the whole plant. This 

 process of mound building can still be seen in isolated hillocks. 

 An examination of the older mounds confirms this theory. In 

 the lower portion of the mound the earth is compact and indu- 

 rated, while the surface soil is a light loam mixed with decayed 

 and decaying leaves. The mound is protected from washing by 

 tbe lains at the summit by the overhanging branches and foliage, 

 and at the ba^e by a compact mass of roots. Outside of the 

 folia,ge and roots the process of erosion goes on steadily, though 

 slowly, during the rainy season, when this soil is peculiarly sus- 

 ceptible to the action of water, and the hollows between the 

 mounds are then formed. 



When in the course of time the plant dies from natural decay, 

 from being smothered by the drift that environs it or from the 

 fires that sometimes sweep over these plains, the mounds, being 

 deprived of protection, are attacked bj' wind and rain and gradu- 

 ally worn down. The mounds are thus made shallower and 

 broader at the base, until from this steady subsidence they sink 

 down and flatten out almost to the general level of the plain. 



Tbe presence of living shrubs upon the more perfect mounds 

 and of masses of roots well preserved or in process of decay 

 in mounds in subsidence, where no large growing vegetation has 

 been seen for many years, and in the oldest and flattest mounds 

 the disappearance of all traces of shrubs and roots, confirm our 

 theory of mound formation and subsidence. 



What the shrubs I have named — Rhus, Simmondsia and Iso- 

 meris — have effected in cooperation with the wind and rain 

 in the formation of mounds in this section, has been accom- 

 plished elsewhere by other shrubs and trees. It is a familiar 

 fact that upon the great prairies of Texas mats of timber are 

 generally found upon the summit of hillocks, very much larger, 

 of course, than the mounds of southern California, as those trees 

 are larger than our shrubs. 



CURRENT NOTES ON ANTHROPOLOGY. — XXXI. 



[Edited by Z», G. Brinton. M.D., LL.D., K.Sc] 

 The Archaeology of Oaxaca. 



Two or thive years ago the State of Oaxaca, in Mexico, es- 

 tablished an Archceological Museum, and placed it in charge 

 of the very competent and enthusiastic scientist. Dr. Nicolas 

 Leon, of Michoacan, who had aheady won for himself a wide 

 reputation as curator of the Museum at Morelia. Through some 

 unfortunate political changes the modest appropriations awarded 

 to both these institutions have been diverted into other channels. 

 This is a matter of great regret to all who are interested in the 

 preservation of the ancient monuments of Mexico and the fur- 

 ther investigations into the numerous remains there found. 



The State of Oaxaca especially has an archaological impor- 

 tance which attaches a unique value to the investigation of its 

 remains. From the earliest days of which tradition records the 

 echoes, it was the home of the Zapotecs, and the profoundest re- 

 searche-: into the pre-Columbian origin of the Aztec and Mexi- 

 can civilization point, not to the fabulous '• Empire of the Tol- 

 tecs," but to these Zapotecs as the tribe which first spread abroad 



