MtCiEEj THE ABUNDANT AQUATIC LIFE 39 



aixl more than ordinary littoral variety; for the waters of the gulf are 

 warmed by radiation and conduction from its sun-parched basin, while 

 the concentrated tides distribute and stimulate the species and keep 

 the vital streams astir. 



Local Features^ 



Considered as a tribal habitat, Seriland comprises four subdivisions 

 of measurably distinct character, viz, (1) the broad desert bounding 

 the territory on the east; (2) the mountainous zone of Sierra Seri; (3) 

 Tiburon island and the neighboring islets ; and (4) the navigable straits 

 and bays contiguous to island and mainland. 



1. So far as its marginal portions are concerned, Desierto Enciuas 

 is a ty])ical valley of the Sonoran province, sparsely dotted with vital 

 colonies of the prevailing type and variegated by the exceptionally 

 luxuriant mesquite forests of the I5acuache and Sonora fans; but the 

 interior of the valley is rendered distinct by the fact that it lies near, 

 if not below, the level of the sea.' The central feature is Playa 

 Noriega — a lilm of brackish water for a few days after each consider- 

 able semiannual freshet, a sheet of saline mud for a few weeks later, 

 and for the greater ])art of the year a salt-crusted sherd 20 square miles 

 in area, level as a floor and unimpressionable as a brick pavement. 

 The playa is rimmed by dunes 10 to 40 feet in height, and about these 

 and along the arroyos which occasionallj' break into it there is some 

 aggregation of salt-enduring shrubs, evidently sustained in part by the 

 semiannual freshet with its meager vajjors and fogs. Outside this 

 rim the surface is exceptionally broken; low dunes and irregularly 

 wandering banks of soft and dust line sand are interspersed with 

 meandering salt flats much like the central playa, I'anging from a few 

 feet in width and a few yards in lengtli up to mappable dimensions, as 

 in the lesser playa lying east of the great one; and many of the dust- 

 banks are honeycombed with squirrel burrows. This annulus of broken 

 surface is narrow on the west, soon passing into okatilla scrub and then 



'The expedition of 1895, during which Seriland was surveyed, was not provided with apparatus 

 for a*-cnrate vertical nieasareiiient, and hence altitudes were only approximately detertiiined. The 

 determinations by Mr Johnson, who executed the topographic surveys, indicated that even the 

 lowest part of the valley is somewhat above sea-level; but other facts indicate that it actually lies 

 below the level of the watersof the gulf, and forms a miniature honiologueof Colorado desert (in south- 

 ern California) : in the tirst place the central playa, which is undoubtedly flooded occasionally if not 

 semiannually, does not embouch into, and has no channels extendini; toward, the sea; in t!ie second place 

 it is hifihiy saline; again, the alluvial fans of Rio Baciiache and (especially) of Rio Sonora are so 

 placed as to intercept and dam the trough occupied by Laguna la Cruz in its southern portion, and 

 ria,\ a Noriega in its northern jiortion ; concordantly, the detail configuration of the coast indicates 

 nuirine transgression, a])parently due to secular subsidence of the land — thougli the abundant marine 

 shells of recent species t(tward the valley -bottom attest recent displacement of the sea. On the whole, 

 the facts seem to indicate that, during recent geologic times, the lower portion of this valley was a 

 shallow gulf extending northward (and ]irobably also southward) from the eastern limit of Baliia 

 Kino; that the importation and deposition of sediment, chiefl.v b.v Rio Sonor;i, outran the secularsub- 

 sidence of the land so far as to displace the watersof tlie gulf in its central portion and j.o separate the 

 northern arm from the sea; an<l that the w;tters of this northern arm were subsequently evaporated, 

 disappearing tiniilly in Ihe central playa in which local inflow and evaporation are balanced by the 

 usual mechanism of interior basins. 



