aughan.] PHYSICAL FEATURES. 15 
|g to the Leguminosse, Rhamnacaae, and Cactacae, constituting the 
taparral of the Mexicans. There are enormous orchards of Opuntia 
afinesquei, the common prickly pear or nopal of the Mexicans, and 
asajillo, Opuntia l&ptocaulis, is often extremely abundant. Of the 
pguminosse, the mesquite and several species of acacia 1 are the most 
on in ion. Lignum vitse also is a common and characteristic plant. 
Except along the creeks, where occasionally live oaks grow, there are 
10 trees, unless the mesquite reaches a size sufficient to entitle it to 
hat name. Professor Bray, of the University of Texas, is making a 
areful study of the flora of western Texas with reference to climatic 
nd other natural conditions. Rains are very infrequent, and the 
ountry is not capable of supporting a dense population. 
The following table gives the average annual rainfall at some of the 
lore important places, 1 as determined by observations extending 
tough several years: 
Inches. 
Fort Clark (Brackett) 24. 02 
. Fort Inge (Uvalde) 25. 35 
Eagle Pass 25. 01 
Laredo 19. 58 
The mean annual temperature is as follows: 
Degrees (F). 
Fort Clark 68. 8 
Fort Inge 68. 7 
Eagle Pass 72. 
Laredo 72. 9 
January is the coldest month, but the average temperature for that 
onth at any place is rarely as low as 45°. August and July are the 
latest months, but the average for these months is rarely as high as 
°. In the summer, in the middle of the day the heat is intense, but 
night there is always a refreshing breeze. 
DESCRIPTIVE GEOLOGY. 
The region as a whole is underlain by a series of rock sheets 
longing to the Cretaceous and Tertiary periods, which are covered 
ia considerable extent by surlicial deposits of silt and gravel. The 
is will now be described in ascending order. 
CRETACEOUS. 
RIO ORANDE SECTION. 
n he Cretaceous strata of the Texas region have been divided into 
) great subgroups or series, an upper and a lower. These series 
e been further divided into formations, which are still further sub- 
isible into individual strata; but for purposes of general discussion 
% formations are the units of the system of classification. 
imatic conditions of Texas, by Gen. A. W, Greely : Senate Ex. Doe. No. 5, Fifty-second Congress, 
