discovered. Silurian(?) slates are found at Guanajuato, Catorce, and Jacatecas. The Catorce slates are Jurassic. Devonian. Aguilera looked up the reported Dev. localities but in each case found no post-Jurassic rocks. Carboniferous. Are found beneath the Gulf. along the Guatemalan border. Limestone with P. seminecticalatus. Fragra and Oaxaco's Cath. localities are now known to be within Gulf or J. outcrop age. Aguilera thinks the Archean ridge continued to persist as islands were through the Pal. alienation and other united into a Jurassic ridge at the Perm-Carth. movements. Upper Triassic. Hallas' crust deposits along Gulf coast up to 3000 feet, 1600 feet in Sonora. Sandstone and slates. Rest on Archean, or fossils. The strata are not thought to be momentum that for quite late Triassic formation but may be extreme deposits. Jurassic. Ingression of the sea. These rocks go with the Cretaceous and are folded with them. Lower Jurassic in southeast Mexico. Upper Jurassic in