Carboniferous Limestones.
The Carboniferous limestones form high ridges parallel with the general course of the Sierra Madre, from Hermosillo, north of Guaymas, east of Sahuaripa. These ridges become more elevated as we approach the crest of the Sierra. The rocks of this formation are fine-grained and bluish in color, and form heavy beds with intercalated schistose layers; they contain nodules and beds of flint. There are some clay slates at the base of the formation. The thickness of the whole series is probably over five thousand feet. The principal localities where these Carboniferous rocks may be observed are as follows, naming them in order from west to east:
1st. Hermosillo, where they rest on syenitic granite and are highly metamorphosed, the limestones being converted into white saccharoidal marble, and the slates into garnet and epidote rock. Dykes of green porphyry cut through the beds of sedimentary rock, which beds have a strike of about N. 65° W., and stand nearly vertical.
2d. Five leagues from Hermosillo, at La Cruz; in the Cerro de Santa Teresa on the south, and the Sierra de Las Animas on the north. Here the limestones contain crinoids.
3d. Four leagues farther on, between La Noria and El Aguajito; here are high granite ridges with a granite axis.
4th. Twenty leagues from Hermosillo, south of Ures; Carboniferous rocks upheaved on the southwest side of the granitic Sierra de Mazatan. The direction of this range is from northwest to southeast, and its height sixteen hundred varas, according to M. De Fleury; here are a few silver mines.
5th. Haciendita, nine leagues farther northeast. The beds here are metamorphosed and much disturbed, dipping northeast; these outcrops form low hills.
6th. Between Mátape and Batuco; a very high ridge of granite, running in a northerly direction, with limestone resting upon it. To the north and east of Topisco the limestones attain a great thickness and afford fine fossils. At the Cerro de la Bonacina, one of the highest points of the range, a variety of corals, crinoids, and brachiopods may be seen weathered out from the surface of several beds of hard, compact limestone, of various colors; these beds are near the summit of the mountain. This locality was first discovered by Don Antonio Moreno, Engineer of the Bronces mine. The strata here are much disturbed, and appear to have been folded into a mass with a synclinal structure.[23]
Triassic Rocks.
This formation is usually highly metamorphosed, and passes into porphyries at its base. The strata are more or less inclined, and the lower beds are very much contorted and disturbed. The rocks referred to the Trias extend from Soyopa to San Javier; but they are developed on a more extensive scale between San Antonio de la Huerta and Los Bronces, forty-two leagues northeast of Guaymas. The Triassic rocks form a chain of high and rugged mountains extending from south-southeast to north-northwest. The isolated mining districts of Tecoripa and San Marcial (between Los Bronces and Guaymas) are in the same formation; it also crops out from under the stratified volcanic rocks at the Punta de Agua, between San Marcial and Guaymas. The metalliferous greenstones and porphyries, previously noticed, form the nucleus around which the Triassic beds have been upheaved. These beds are seen near San Javier and Los Bronces, two mining towns which are situated on greenstone, but which skirt the foot of a small ridge of feldspathic porphyry, much less elevated than the metamorphic rocks themselves. They are also seen overlying granite, near the Cerro Colorado, between Soyopa and Los Bronces, and south of Tecoripa. The Cerro de la Nahuila, the highest point but one in the district, lies southeast of the Sierra de Mazatan. There are three principal divisions of the Triassic, which occur in the following order, the first mentioned being the lowest: