84 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
canons of the headwaters of various Texan rivers and their tributaries. 
In the south it is prolonged farther to the eastward than in the north, 
and conforms here pretty closely to the 2000 feet contour line. 
LOWER SONORAN. 
Arizona.— I do not consider that there is any actual Lower Sonoran 
whatever in New Mexico, that can be classed as such. In Arizona this 
zone follows rather broadly the valley of the Colorado river up to south¬ 
ern Nevada. Here it gradually narrows and finally becomes confined to 
the bed of the canon, extending up in a dilute condition as far as the 
mouth of the Colorado Chiquito and into Marble Canon. It also extends 
up the canon of the Colorado Chiquito, and embraces to a greater or 
less extent the lowest parts of the Painted Desert contiguous thereto. 
In the paper on the Grand Canon already referred to, the belt called 
“Gila” (2500 to 4500 feet) is dilute Lower Sonoran. 
The Lower Sonoran also very broadly follows the Gila up to a point 
well above Phoenix and Florence, spreading out in its lower course to 
join the Lower Sonoran of the lower Colorado valley, which on the other 
side of that river extends into the Mohave region and still farther west¬ 
ward into Central California. Farther up the Gila and its confluents, 
the Lower Sonoran narrows and is then confined to the lowest parts of the 
valleys or canons, but falls short of the New Mexico line. In southern 
Arizona it extends eastward to Tucson and vicinity. 
Mexico. —From near Tucson the eastern boundary of the Lower Son¬ 
oran extends in a general southerly direction into . Sonora, then south¬ 
easterly to a little north of San Ignacio, and soon afterward strikes the 
Sierra Madre, where it becomes identical with the lower limit of the Up¬ 
per Sonoran. It enters the Sierra Madre on the western side of the range 
at various points, following up the larger canons, notably the Grand 
Barranca of the Urique, a gorge which is destined to rank among the 
famous scenery of the North American continent, when the Sierra Ma- 
dres of Mexico become better known. 
The northern limits of the Lower Sonoran in Mexico are of course the 
same as already described in general for the southern limits of the Upper 
Sonoran. Its southern, or rather southeastern, limits are in the Isthmus 
of Tehuantepec. It takes in the Guadalajara region, and extends all the 
way from Puebla to Oaxaca, along the line of the Mexican Southern 
Railway. It is lost to the westward of this latter region in the more 
humid lowland-valley region directly south of the City of Mexico, but 
its western boundary farther north approaches gradually nearer the sea, 
taking in all but the lowest land, and reaching the sea coast atGuaymas. 
On the east it follows the irregular edge of the plateau line northward to 
