312 PROCEEDINGS OF THE AMERICAN ACADEMY 



The Canadian River is a southern tributary of the Arkansas, run- 

 ning eastwardly very nearly under the 35th degree of latitude, and 

 bounding on the north the elevated plains known as the Llano Esta- 

 cado, in the northwestern parts of Texas and the adjoining regions of 

 New Mexico. 



The Upper Rio Grande runs through New Mexico from north to 

 south ; the capital, Santa Fe, is not far from the river, in lat. 35|-° ; 

 and the town of Albuquerque is a little below. Doiiana is a small 

 place on the river, above El Paso. El Paso itself, where the Rio 

 Grande breaks through the mountain ranges, changing its heretofore 

 southern to a southeastern course, is the central point of our Cactus 

 region, partly from its geographical position, and partly because many 

 of our explorers have made it the centre of their operations. 



The present southwestern boundary of the United States runs from 

 El Paso irregularly westward through the former Mexican State of 

 Sonora, to the Colorado " of the West," or " of California," which 

 comes from the South Pass in the Rocky Mountains, and runs south- 

 westward and southwardly. Its principal tributaries rise in the east ; 

 those most important to us are the Little Colorado or Colorado Chi- 

 quito, under the 35th and 36th degree of latitude ; Bill Williams's 

 Fork, or Williams's River, as it is lately styled, further south ; and 

 in lat. 33° the Gila River, which rises near the " Coppermines," north- 

 west of El Paso. 



Proceeding from Santa Fe westward, we find the Indian town of 

 Zuni, on the head-waters of the Little Colorado ; then the San Fran- 

 cisco mountains ; the Cactus Pass, at the head of Williams's River, 

 and this stream itself. All this territory is at present included in the 

 political organization of New Mexico, though uninhabited by whites. 



West of the Colorado, in lat. 35°, is the Mojave or Mohave River, 

 ■rising in the Sierra Nevada near the Cajon Pass ; lower down, oppo- 

 site the mouth of the Gila, the country is a sandy desert extending 

 westward nearly to San Felipe, on the eastern slope of the California 

 mountains in the same latitude. On the western sea-coast the town of 

 San Diego is the only interesting point for the plants under review. 



Geographical Distribution of the Cactacece in the Territory of the 



United States. 



As to the geographical distribution of the Cactacece, our territory 

 may properly be divided into eight regions, viz. : — 



