32 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



mixed with effusive rhyolite and basalt. In the Coast Range the 

 granite is of a finer quality and associated with gabbro-diorite and 

 hornblende-mica-andesite, to which, on the Pacific coast and San 

 Clemente Island, augite-andesite is added. 



FLORA OF THE MEXICAN BOUNDARY REGION. 



The flora of the lower Rio Grande Valley is characterized by a 

 distinct tropical element. The Tamaulipan fauna, which mixes with 

 the Texan along the lower course of the Rio Grande, is associated 

 closely with the range of the Texas palmetto {I nodes texana Cook). 

 This tree grows in the rich soil of the bottom lands from the neigh- 

 borhood of Hidalgo nearly to the Gulf, and below the Rio Grande 

 it ranges southward in the neighborhood of the coast to southern 

 Mexico. The bald cypress (Taxodium distich tun) is another tree 

 of the humid region. Its range extends up the valley of the Nueces, 

 and a few trees have been found between that and Devils River. 

 Beyond the general range of the palmetto and bald cypress scarcely 

 any of the tropical mammals and birds of the Tamaulipan fauna 

 exist. 



In the flora of the narrow strip of country which lies between Fort 

 Clark and Devils River many eastern species are represented. The 

 region included between Devils and Pecos rivers marks the transi- 

 tion from the Texan flora to that of the dry interior region, which 

 extends from the Pecos River, in southwestern Texas, west to the 

 Coast Range Mountains of the Californias. 



From the summit of the Coast Range to the Pacific Ocean the flora 

 is almost pure Californian. 



The broad reach of country lying between the Pacific Coast and 

 Middle Texas tracts, constituting the arid interior region, belongs 

 principally to the Lower Sonoran or Austral Life Zone. In the moun- 

 tainous portions of this dry interior region a flora analogous to that 

 of the Rocky Mountains is introduced, which, however, is highly 

 colored by forms extending northward from the highlands of Mexico. 



Considered as a whole the Mexican boundary strip is, compara- 

 tively speaking, a treeless region. Forests are mainly confined to 

 the mountain ranges and the vicinity of streams, which latter are 

 few and of insignificant size. On some of the desert areas arbores- 

 cent cacti and yuccas form open groves. The streams are regularly 

 lined with trees, of which the Fremont cottonwood. black willow, 

 boxelder, walnut, sycamore, oak, mulberry, ash. and wild china 

 trees are usually the most abundant. Of these the cottonwood and 

 willow are almost certain accompaniments of every permanent 



« See Petrographic Report on the Rocks from the United States-Mexico Bound- 

 ary, by Edward C. E. Lord, in Proc. U. S. Nat. Mns.. XXI. 180!). \n>. 77. -, .-7S2, 



pi. LXXXV. 



