86 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



deposited arkose. In this locality also, trending north and south, may 

 be seen a southern segment of the Sierra GIganta fault, which Itself 

 appears to be a continuation of the San Leandro fault zone, so well 

 known In '*Alta California." The sediments of this locality show that 

 this low section of the peninsula did not arrive above the sea until the 

 Pleistocene. The adjacent part of the peninsula, the Sierra GIganta 

 anticline, is composed largely of Pliocene marine sediments. This all 

 Indicates that the block of Igneous rock In the Cape District had a long 

 existence In the Tertiary as an island. This Is significant in the con- 

 sideration of the biota and especially the flora. Many plants are known 

 only from the Cape District and the adjacent peninsula, to which latter 

 area they have migrated In Quaternary times. I propose to call such land 

 bodies as the Cape District, postinsular. 



Besides the Cape District there are many coastal "cerros" (a word 

 which the Mexicans apply to their craggy hills), whose positions and 

 the adjacent landward strata indicate are also postinsular mountains. 

 It Is not within the scope of this paper to present detailed evidence for 

 this Interpretation of the physiography, but the facts : that post-Pliocene 

 detrital and alluvial materials from the Sierra Madre Occidental have 

 built up the coastal plains from 100 to 500 feet; that the Pacific coast 

 is rising; that higher sea levels existed universally in the Interglaclal 

 periods; and that several Quaternary estuarlne deposits and sea caves 

 exist several miles Inland, all provide excellent grounds for this theory. 

 More detailed evidence accumulated during my several years of paleon- 

 tologlcal reconnaissance In the area, will, I hope, appear in a later study. 

 The larger coastal mountains which may be considered postinsular are; 

 in Sonora, Sierra Coloral, Sierra Serl, the monadnock north and west 

 of Guaymas, probably Sierra Bacatete, Sierra Bojihuaqueme, and in 

 Sinaloa, Sierra Navachlste, Sierra Tecomate, and possibly Sierra Ta- 

 culchamona. 



The floras of these postinsular localities have not been Individually 

 studied. On most of them few if any collections have been made. More 

 collections have been made on the Guaymas monadnock than on any of 

 the others, but they are vv^Idely scattered In herbaria and not available 

 for detailed study. Nor do they represent all of the postinsular land 

 body lying northwest of Guaymas. Beginning with the collections of 

 Thomas Coulter in 1829-30, the plants taken at Guaj^mas and vicinity 

 have disclosed a surprising number of novelties, some of which presum- 

 ably had their specific origin on the Guaymas monadnock. The post- 

 insular Sierra Coloral has not been studied by botanists. McDougal, 



