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. 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: