262 Scientific Intelligence. 
To the north, the are east of Sahuaripa are also very high; but they 
have never been measured. No peaks or ridges, however, in this por- 
tion of Mexico attain anything like the elevation of the higher portion 
of the Sierra Nevada, few if any points ean 4 10,000 feet i in altitude. 
“The direction of the Sierra is nearly that of a line connecting seme 
of the best mining districts in Mexico, a hiok. are situated on or very near 
the summit of the mountains. These districts are the following, enumer- 
ating them in their geographical order from the south toward the north ; 
In Durango, San Antonio de las Ventanas, Guarisamey, and San Dimas, 
remarkable for their auriferous silver ores ,and sixty-two Mexican leagues 
northeast of Mazatlan; in Chihuahua, Guadalupe y Calvo, and San Pe- 
dro de Batopilas, yielding fine specimens of native silver; also, Jesus 
arie, in the same State, and the Real del la Cieneguita, Sonora, with 
aitver ‘and gold mines. 
“The geological structure of the occidental slope of the Sierra Madre, 
as well as that of the other parts of this great chain, is exceedingly in- 
teresting, and, as yet but little known, oerrbetaiding the valuable in- 
_ vestigations of Humboldt and other eminent men; for, up to the present 
time, , the age of the different formations has never been fixed with any 
degree of accuracy, from want of materials and of sufficient observations. 
In 1863, 1864, and 1865, however, I explored quite a number of locali- 
dikes of extremely varied character. The granites, however, are ve 
poor in veins of the precious metals, while the porphyries are highly 
metalliferous, In Sinaloa (Candelero) and Durango (San Dimas) we see 
@ granites underlie the metalliferous porphyries, and that the 
greenstones, in Sonora (near Hermosillo and in the vicinity of La Ha- 
ce a penetrate through them 
“The oldest sedimentary rocks, “which I have observed, belong to the 
arboniferous series; this is represented in the eastern part of set ple 
masses of ‘limestone, forming baal high and meer rid -e 
paving a little west of north. The upturned strata are seen, any 
s, to rest on granite. Argentiferous veins occur Shecmetions wi 
formation. 
“The next pad of Ege smal rocks, in order, is the Triassic; this 
