SOUTHERN PACIFIC LINES 131 
a wide region extending from western Texas to southern California. 
The winters are mild, and although the summers are hot the air is so 
dry that the heat is much more endurable than in the sultry periods 
that occur in the summers of the Eastern and Central States. Decem- 
ber is usually the coldest month, with an average temperature of about 
40°. On nearly 300 days in a year there is sunshine for the greater 
part of the day, and storms of long duration are rare. The region 
lies outside of the normal storm track that extends over the central 
United States, and in consequence the weather is much more uniform 
than in the regions farther north and east. The principal rainy 
season is in July, August, and September. The annual rainfall in the 
wide valleys is mostly less than 10 inches, but on the higher ridges 
there are many rains and snows at times when there is little or no 
precipitation in the adjoining desert valleys. 
NORTH LINE FROM EL PASO, TEX., TO MESCAL, ARIZ. 
From El Paso westward to Tucson the Southern Pacific Railroad 
has two lines—one going by way of Deming and Benson and the other 
(formerly the El Paso & Southwestern Railroad) by way of Columbus 
and Douglas. Leaving El Paso, the north line of the railroad follows 
the north bank of the Rio Grande for some distance, with Mexico in 
plain view on the opposite bank. (Turn to sheet 18.) In about 1 
mile a large smelter (pl. 18, A) is passed, and in 2 miles a cement plant, 
near which are large quarries in limestone (of Comanche or Lower 
Cretaceous age) in a downfaulted block at the south end of the Frank- 
lin Mountains. It is the presence of this rock and a mass of intrusive 
porphyry associated with it that causes the constriction of the river 
valley at El Paso (Spanish, the pass). At the entrance of this pass 
the railroad crosses the river into New Mexico and skirts the north 
side of the Cerro de Muleros, a high ridge which lies mostly in Mexico. 
At one place the railroad is within a few yards of the Mexican bound- 
ary, and one of the monuments is plainly visible on a hill near by.® 
The Cerro de Muleros consists of a mass of limestone, shale, and 
sandstone of Cretaceous age penetrated and tilted by a large in- 
trusive stock of porphyry.% The lower or quarry limestone in this 
succession (of Fredericksburg, Lower Cretaceous age) is well exposed 
% The monument marking the be- 
e 
bank of the Rio Grande a short distance 
south of the west end of the south-line 
railroad bridge across the river. It in- 
dicates the location of the point near by 
where the ‘‘deepest channel of the Rio 
Grande touches the parallel of 31° 47’ 
north latitude,’’ as prescribed in the 
terms of the Gadsden Purchase. 
p. 152.) 
(See | 
% In his description of the Cerro de 
Muleros, Bése gives the following sec- 
tion of the Comanche stratigraphy: 
1. Limestone, hard, white and 
light gray, with Exogyra Feet 
WW 4 - 
2. Marl, yellow, with Exogyra 
whit and Hemiaster 
Mei ee. 
3. Sandstone, red-brown, thick 
bedded, wi 
w, i ts 
