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.