The most recent formation is that of the terrace deposits of sand and gravel, which occur in Sonora.
Having thus given a general sketch of the principal groups of rocks developed in the region in question, I pass to a more detailed description of the different formations.
Granites.
Underlying all the rocks in Durango and Sinaloa, and probably posterior to the Carboniferous limestones, which they have in places extensively metamorphosed, are masses of granite. These may be seen in many places between the coast and San Dimas, either occupying the bottoms of the valleys, or forming independent hills. There are two well-marked varieties: Of these the first are syenites, more or less fine-grained, and consisting of a mixture of feldspar, variously colored, quartz, black or green hornblende, and black or brown mica, the latter usually in hexagonal plates. Localities of this variety are: Haval, Las Higueras, San Ignacio, Santa Apolonia, Candelero, La Noria, Zaragoza, etc., in Sinaloa; San Marcial and Tecoripa valleys, Hermosillo, in Sonora. The other variety is either very coarse-grained, consisting of white feldspar, gray quartz, and plates of silvery mica, or else finer grained, and chiefly made up of feldspar; these occur, forming mountains and ridges in Sonora, in the Sierra del Amolé, near La Magdalena, Sierra del Espinaso Prieto, near Hermosillo, and the Sierra de Mazatan, south of Ures. The fine-grained granites contain argentiferous veins at Zaragoza, in Sinaloa, and east of Topisco, in Sonora. These are traversed by numerous intersecting dykes of diorite, feldspar, and quartziferous and feldspathic porphyries, especially well seen near Hermosillo, and the Cajon de los Carrisos, east of San Antonio de la Huerta. There are no metalliferous veins where the granite is thus intersected by dykes.
Metalliferous Porphyries.
These may be divided into two groups. The first consists of a rock occurring in large irregular masses or beds, and having a dark colored argillaceous base, through which are disseminated small crystals of whitish feldspar. This variety, which is probably older than the granites, includes some of the richest mines of the Sierra Madre; as those of Candelaria, Bolaños, Cinco Señores, etc., near San Dimas, in Durango; and which have yielded over $20,000,000. There are also rich veins in this kind of rock at Candelero, fifty-two leagues northeast of Mazatlan, in Sinaloa. All these veins run northeast and southwest, and are cut at right angles by dykes. The second variety of porphyry is a gray feldspathic rock, apparently made up of labradorite and magnetic iron ore; this overlies the greenstones, and is covered by the Triassic beds at Los Bronces and San Javier, where there are three systems of argentiferous veins. The Nahuila mine, one of the best in Sonora, is in this rock.
Metamorphic Rocks.
Heavy masses of metamorphic rocks may be seen at various localities in Sinaloa and Durango (Tecomate, Tenchoquelite, Arroyo del Ciruelo, Arroyo de San Vincente) resting either on the granites or the metalliferous porphyries. These rocks occur in masses or beds, sometimes distinctly stratified, and sometimes without any traces of the original bedding. They are always much altered and broken up. Their lithological characters are not well marked, although the series is easily recognized. The rocks referred to in this division, are usually fine-grained, of a greenish or bluish color, when not too much decomposed, and somewhat argillaceous in composition. At the base they pass into porphyries. The argentiferous veins cut both the metamorphic and the porphyritic rocks at Tecomate, on the Rio de San Ignacio, where the dip of the formation is to the northeast, at an angle of 70°. Between La Puerta and El Pilar, Arroyo de San Dimas, they occur in jaspery layers, ribboned with green and brown hues, and resemble some of the metamorphic Triassic rocks of Sonora. Near Candelero, the metamorphic rocks are associated with whitish, semi-crystalline limestone. The formation in question may be observed in many other localities in Sinaloa, always resting on granite and passing into porphyry; it is also sometimes associated with metamorphic slates.
Greenstones.
These rocks occur in heavy masses or in beds, and are made up of a fine-grained, compact mixture of hornblende and feldspar, often containing mica, and having a greenish color. The greenstone underlies the Triassic rocks, and in many places it protrudes through the granite. This rock is highly metalliferous at Copála, Sinaloa, and also at Los Bronces and San Javier, in Sonora. The greenstones or diorites which occur in the granite, appear to be anterior to the metalliferous greenstones, and the latter are posterior to the Triassic.