This formation is usually highly metamorphosed, and passes into porphyries at its base. The strata are more or less inclined, and the lower beds are very much contorted and disturbed. The rocks referred to the Trias extend from Soyopa to San Javier; but they are developed on a more extensive scale between San Antonio de la Huerta and Los Bronces, forty-two leagues northeast of Guaymas. The Triassic rocks form a chain of high and rugged mountains extending from south-southeast to north-northwest. The isolated mining districts of Tecoripa and San Marcial (between Los Bronces and Guaymas) are in the same formation; it also crops out from under the stratified volcanic rocks at the Punta de Agua, between San Marcial and Guaymas. The metalliferous greenstones and porphyries, previously noticed, form the nucleus around which the Triassic beds have been upheaved. These beds are seen near San Javier and Los Bronces, two mining towns which are situated on greenstone, but which skirt the foot of a small ridge of feldspathic porphyry, much less elevated than the metamorphic rocks themselves. They are also seen overlying granite, near the Cerro Colorado, between Soyopa and Los Bronces, and south of Tecoripa. The Cerro de la Nahuila, the highest point but one in the district, lies southeast of the Sierra de Mazatan. There are three principal divisions of the Triassic, which occur in the following order, the first mentioned being the lowest: { { Quartzites and clay slates; { { Black, jaspery schistose layers; { 1.{ or, where the rocks are less altered: { { { Black clay shales with beds of coal; { { Argillaceous sandstones. 2. Quartzites, in great thickness. 3. Heavy beds of conglomerate. The interstratified clay shales and grits of the lower member, crop out in several places along the Cañada de Santa Maria, at the bottom of the ravines below Los Bronces. Here, there are three or four beds of good anthracite coal, with a considerable number of well-preserved plants occurring in the associated clay shales, both above and below the coal.