ACADEMY OF NATURAL SCIENCES. 245 



The name of the " Sierra Mad re " is usually applied to the main range of 

 mountains of this country, or the western border of the plateau which stretches 

 north through the territories of the United States, forming what may be called 

 the great orographical feature of the continent. In Northwestern Mexico this 

 crumpled border of the great plateau comprises an extensive mountainous re- 

 gion, by no means forming a continuous single chain, but rather several central 

 ranges, with associated groups of parallel ridges, all having the same geueral 

 course, which is approximately north-northwest, aud south-southeast. As the 

 breadth of the chain widens as we go towards the north, so, too, that of the 

 valleys increases in that direction, the whole system of mountains and valleys 

 spreading out in something like a fan shape. 



Going north, the chain appears to sink gradually, although determinations of 

 altitude in Northern Mexico are extremely few in number. It is certain that 

 there is, in about latitude 32°, a depression of the mountain ranges which ex- 

 tends entirely across the continent, and which would enable the traveler to 

 cross from the Atlantic to the Pacific, without necessarily surmounting any 

 elevation greater than four thousand feet.* The southeastern range is the high- 

 est, and the culminating point is said to be the Cerro de Cuiteco, sixty leagues 

 northeast of Jesus Maria, on the western border of Chihuahua. The approxi- 

 mate altitude of the Cumbre de Basascachic is seven thousand four hundred 

 and twenty-nine feet, and that of Guadalupe y Calvo, seven thousand eight 

 hundred and twenty-five feet. To the north, the ranges east of Sahuaripa are 

 also very high ; but they have never been measured. No peaks or ridges, how- 

 ever, in this portion of Mexico attain anything like the elevation of the higher 

 portion of the Sierra Nevada, few if any points exceeding ten thousand feet in 

 altitude. 



The direction of the sierra is nearly that of a line connecting some of the 

 best mining districts in Mexico, which are situated on or very near the summit 

 of the mountains. These districts are the following, enumerating them in their 

 geographical order from the south towards the north : In Durango, San An- 

 tonio delas Ventanas, Guarisamey, and San Dimas, remarkable for their aurif- 

 erous silver ores, and sixty -two Mexican leagues northeast of Mazatlan ; in Chi- 

 huahua, Guadalupe y Calvo, and San Pedro de Batopilas, yielding fine speci- 

 mens of native silver ; also, Jesus Maria, in the same State, and the Real del la 

 Cieneguita, Sonora, with silver and gold mines. 



2. General Geology. 



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 interesting, and, as 

 yet but very little known, notwithstanding the valuable investigations of Hum- 

 boldt aud other eminent men ; for, up to the present time, the age of the differ- 

 ent 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 localities in northwestern Mexico, and was thus 



*See Emory, in Mexican Boundary Report, vol. 1, page 41. 



