NO. 2 gentry: land plants 97 



extensive habitat, which show considerable variance according to the 

 degree of sun and wind exposure. Such rare plants as Perityle Pabneriy 

 Asclepias leptopus, and Dcsmodium Wigginsii have been found along 

 the cliffs, as well as many cacti, Agave, and Ficus Palmeri. Conglomer- 

 ates are apparent and the Recent alluvial deposition of clay and silt has 

 tended to lap upon the mountain sides. The present cycle of degrada- 

 tion at work in the area, however, exposes and sorts the coarse and fine 

 gravels along the shallow arroyo channels, or hurries the coarse frac- 

 tured rocks down the mountain slopes and canyons. 



Plants that attain true tree stature are limited to the run-off chan- 

 nels or the margins thereof. Saline littoral flats are extensive and an 

 unusually rich assortment of halophytes make up a complex association, 

 and among which are commonly found the following genera: A triplex^ 

 Lycium, Atamisquaea, Zizyphus, IVisUzenia, Salicornia, Prosopis, Steg- 

 nospennum, Rhizophora, and many others. The sandy beach lines are 

 widely and intricately interrupted by the steep rocky slopes and cliffs 

 that are hammered by the palpitating sea. On the landward side of the 

 area, alluvial materials have been banked high upon the mountain pedi- 

 ments forming broad plains of Pleistocene aggradation. 



The strata are dominated by volcanics which have intruded and 

 overlaid sedimentaries and are in turn partly overlaid by Quaternary 

 beach and littoral deposits. The process of deposition on the bajadas and 

 a structural uplift have raised the area above the confines of the sea. 

 "The hills about Guaymas and for about 80 kilometers to the north 

 are volcanic, consisting of basalt, tufa, and agglomerate. In the hills 

 behind Guaymas a number of old sea caves were noted which, though 

 now over 50 meters above the ocean, contained unconsolidated sands 

 and modern shells. This indicates recent movement at least in one sec- 

 tion of the coast" (Johnston 1924:953). The implications of the area 

 being postinsular have been noted above, and while we are not yet cer- 

 tain of this, it is obvious that it has grown in area during the recent 

 period. 



The predominating plant formation of the Guaymas locality is Desert 

 Shrub. However, because it is near the southern limits of the Sonoran 

 Desert (see map in Shreve, !Mallen', and Turnage 1936:215) and per- 

 haps because of conditions imposed by insulation, there are at}'pical ele- 

 ments in the floral composition. The Sonoran Desert Region is bounded 

 on the south by the Sinaloan Thorn Forest and species from this latter 

 formation find their northern limits in or near Guaymas. This com- 

 ponent includes such prominent Thorn Forest species as Acacia cymbi- 



