24 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. [Vol.Yl. 



The arid region south of the Great Basin we propose only in a gen- 

 oral way to refer to. It is one in which there is no barrier to the spread- 

 ing of the same species from the Gulf of Califoi'nia to the Gulf of Mex- 

 ico, and in which the plants of the basin region, of Southern California, 

 of Texas, and of the Mexican plateau and mountains meet and mingle. 

 This district has also a good number of peculiar genera of shrubs : Sal- 

 a.zcD'ia, in Labiatse ; Solacantha, a spiny Simarubacea ; Canotia, ?i rather 

 doubtful Eutacea, to which might be added Thamnosma except that a 

 second species is Texan ; and Chilopsis, which extends into Mexico ; among 

 herbs, Canhya, a singular little Papaveracea ; Petalonyx, in Loasacese 

 (also Cevallia, which extends both to Texas and Mexico) ; SesperocalUs, 

 in Liliacege ; Bithyrcea, which has been joined to the Old World genus 

 Biscutella ; Wislizena and Oxystylis, in Capparidacese ; AchyronycMa, 

 in Illecebracese ; and among Compositse the genera Baileya, Biddellia, 

 Hymenoclea, Hymenothrix ; and here also are the headquarters of Lap- 

 haniia and Perityle. 



3. The eastern icoodless plains. 



If the arid district of the interior of the United States west of the east- 

 ern Eocky Mountains is denominated the region of " Sage Brush" (*. e., 

 of shrubby Artemisia and Chenopods), the mostly less arid, less saline, 

 equally homogeneous, and even more extensive plains between the Eocky 

 Mountains and the eastern forest region may be characterized as the 

 region of Buffalo Grasses. Its full dcA^elopment is between latitude 35° 

 and 45°, where it occupies an average of ten degrees of longitude, ilSTorth 

 of this it is narrowed or interru])ted, and then merges into a district 

 which is woodless from cold or from the nature of the soil, and at length 

 arctic. Southward it is equally broad, and it trends westward and loses 

 itself in the New Mexican plateau region, which has a certain character 

 of its own, but in which the eastern forms of vegetation mingle first with 

 those of the Eocky Mountains, with those of the Mexican plateau, and 

 at length with those which j)revail in the Great Basin. 



The whole region rises very gradually westward and abuts against 

 the mountains at an elevation of, for the most part, fully 5,000 feet. The 

 annual rainfall on its eastern border is from 24 to 32 inches, tolerably 

 well distributed; in its western part 14 to 16 inches. Upon the climatic 

 characteristics, topography, &c., which have been well presented in 

 various reports and summaries, especially in those published by Dr. 

 Hayden, it is not our pui-pose to enter, 



Kor have we here any special call to discuss the vexed " prairie ques- 

 tion," viz, why it is that the eastern border of this broad district should 

 be treeless, except along river banks, even where the annual rainfall is 

 from 28 to 32 inches, and 8 to 10 inches of this in summer — as much rain 

 as is in the upper part of Michigan and on the Canada shore of Lake 

 Huron ; also why prairies exist as deep bays or islands within the Atlan- 

 tic forest region. Suffice it to note that the prairies east of the Missis- 



