At San Antonio de la Huerta, Tecoripa, and San Marcial, argentiferous veins of various ages occur in the lower and middle members of the Trias. At San Marcial, marine (?) shells are found in the clay slates, near the silver mines; [but those which have been obtained, are too imperfect for recognition; they were referred to Mr. Meek for examination. J. D. W.]
At San Marcos, between San Antonio de la Huerta and the Real Viejo, metamorphic jaspery slates occur in connection with the carboniferous limestones; they are probably of Triassic age. Gold is found in the gulches between the quartzite ridges, as in the Cañada de la Higuera, near Los Bronces in the Cañada de la Iglesia, between the latter place and San Antonio de la Huerta, and generally where the quartzites occur.
Jurassic (?) Rocks.
In the eastern part of the Magdalena and Altar districts, are valleys with low hills and ridges of auriferous clay slates, with interstratified beds of porphyry and diorite. Localities of these rocks are Cerros de la Barajita, between Querobabi and Santa Ana; Cerritos de la Tierra Colorada, where the formation contains beds of variously colored limestones, entirely made up of fossils, South of La Magdalena this formation rests on metamorphic sandstones and shales. [The reasons for referring these rocks, with doubt, to the Jurassic formation, have been already given; see page 246.]
Cretaceous Rocks.
In the Sahuaripa Valley, four miles east of Arivechi, and seventy-two leagues northeast of Guaymas, is a locality of Cretaceous Fossils of great interest. The hill in which they occur is called “Cerro de las Conchas,” or “Shell Mountain.” The rocks exposed are unfossiliferous strata of coarse-grained sandstone at the base, overlain by clay shales and argillaceous limestone filled with fossils. The exposure is very limited in extent: the shales are a few hundred feet thick, and they dip to the east, as do also the beds of carboniferous limestone on which the Cretaceous deposits rest. Masses of porphyry crop out from under the shales, without there being any peculiar indication of metamorphism or disturbance in their vicinity. Other patches of shelly rock are said to occur in the valley on the eastern side, at the foot of the sierra. [A small lot of fossils collected here by M. Rémond were referred to Mr. Gabb for examination; and since that, a considerable number of additional specimens have been received, but have not yet been investigated. Several species were identified as already described from Texas, and figured by Roemer in “Die Kreidebildungen von Texas;” these are, Ammonites pedernalis, von Buch; Natica pedernalis, Roem.; Turritella seriatim-granulata, Roem.; Gryphæa navis, Hall; Cyphosoma Texanum, Roem.; Eulima Texana, Roem. Besides these, two other species are identified, namely; Cardium multistriatum, Shum., and Turbinolia Texana, Con.
There is a considerable number of new species among the specimens from this locality, among which the following genera are represented: Turritella, Chemnitzia, Avellana, Cardium, Trigonia, Panopæa, Pinna, Cucullæa, etc. These will be described and figured by Mr. Gabb, who also remarks that the character of the fossils indicates a closer relationship of the formation to the eastern Cretaceous than to that of California.—J. D. W.]
Volcanic Rocks.
Stratified volcanic deposits cover a broad area of the surface between San Ignacio and San Dimas (Sinaloa and Durango); they dip to the west. On the other side of Durango they dip in the opposite direction. The serrated edges of the strata may be seen from a great distance, and are extremely picturesque, towering up like old ruins, their peculiar forms being due to erosion. They are well seen in the Cerro de los Frayles, near Guarisamey, in Durango; visible from Mazatlan. These belong to the second series of volcanic rocks. In Sonora there are three different series of volcanic deposits which form serrated, picturesque, parallel ridges, running north from Guaymas as far as La Magdalena, or over eighty leagues in a straight line. The three main ridges, enumerated from west to east, are, 1, Las Tetas de Cabra; 2, Guaymas de Zaragosa; 3, Range north of the mouth of the Yaqui River. The description of the various volcanic deposits of Northern Mexico may be reserved for a separate memoir, as the number of them is very great.