NO. 2 GENTRY : LAND PLANTS 83 



knowledge of the evolution of land forms is primary. 



The withdrawal of the Mesozoic seas from the Mexican continent 

 was accompanied by widespread uplift. Large areas became land. They 

 included not only what is now the Gulf of California, but also a margi- 

 nal area. During the Cretaceous and early Tertiary the shore lines of 

 Baja California and northwestern Mexico reached 50 or more miles 

 westward of present limits, including some of the islands. This land 

 mass might even have included the Channel Islands on the northwest, 

 Guadelupe on the west, and the Revilla Gigedos to the southwest, al- 

 though there is no real geologic evidence to support such a presumption 

 (cf. Jtn. 1931). However, irrespective of the exact boundaries, it is 

 evident that during the Cretaceous and early Tertiary, Baja California 

 and its gulf were a part of a land mass areally different than exists 

 today. The progenitors of our modern flora had a broad base for develop- 

 ment in arid latitudes; arid because the land lay in a rain shadow of 

 the Mexican continent, barred from the trade winds, and because the 

 weak westerlies blowing onshore are warmed and dried by the radia- 

 tion of these latitudes. 



The climate of the Eocene appears to have been somewhat wetter 

 than the late Mesozoic, which, because of the extensive sandstone and 

 gypsiferous deposits, is judged to have been relatively arid. However, 

 the granite batholithic and pyroclastic intrusions of the late Cretaceous 

 and Eocene must have raised some mountains, which in turn localized 

 climate abetting drought on the one side and decreasing it on the 

 other, according to the mountain orientation to air flow. However, the 

 interiors of the western land areas have apparently been relatively arid 

 since the middle Mesozoic. Since then, if not before, there have been 

 deserts, though the boundaries of them have been modified or shifted 

 according to climatic cycles and to the raising and lowering of land 

 masses. So, at the beginning of the Tertiary in the California Gulf 

 Region, the environment was already diversified and the evolution of 

 modern seed plants well begun. 



In the Oligocene the sea began to invade the old downwarped block 

 of the Southern Pacific Geosyncline for the first time since the Triassic. 

 As the bend deepened the sea invaded the gulf and by the middle Mio- 

 cene reached half way up the present gulf to about Angel de la Guardia 

 Island and Tiburon Island. By middle or late Miocene it was maximal 

 and covered all of the present gulf, the Colorado Desert, and a section 

 of adjacent Arizona and northwestern Sonora besides. Some of the gulf 

 islands apparently date from that time and still persist as the tops of 



