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The Valle de Las Delicias, lying near the geographic center of Mexico, affords outcrops of Permian rocks more than 10,000 feet thick. The entire sequence can be measured in each limb of a truncated syncline. Although predominantly sedimentary, the succession includes much volcanic material in the form of agglomerate and graywacke, as well as thin lava flows and at least two great sills. The sedimentary beds are all marine and are abundantly fossiliferous at many horizons, ammonites and fusulines being exceptionally well preserved.

The faunas indicate direct connection with the Permian basin of West Texas and New Mexico, and this thick section, lying some 200 miles south of the Rio Grande, indicates the course of the Permian geosyncline across Mexico. The detrital sediments appear to be derived largely from the east but the volcanics came chiefly from the west side of the geosyncline.

Local formation names have not been applied, but equivalents of the Leonard, Word, and Capitan units of West Texas are clearly recognized and the highest marine beds of the section at Las Delicias may be equivalent to the lower part of the Ochoa series of New Mexico and, if so, are the youngest fossiliferous marine Permian deposits in America. No Wolfcamp equivalents have been recognized but they may be present in an unexposed interval between the Leonard horizon and the limestones of Los Piloncillos hills, which is believed to be of Pennsylvanian age.

The Permian faunas are in large part described in parts II, III, and IV. During the field work, attention was concentrated on collecting the ammonites and they are undoubtedly the most adequately represented group of fossils. Fusulines are very abundant in certain beds but not all of these are represented in the collections studied. Brachiopods are abundant at certain horizons but commonly are difficult to collect, and the material available is probably a very inadequate representation of the brachiopod faunas. Other biologic groups are so inadequately represented in the collections before us that their study has been postponed until further collecting is possible. Corals and sponges are locally abundant but pelecypods and gastropods appear to be sparse and echinoderms extremely rare.

Fusulines are represented by 5 genera and 11 species, of which 1 genus and 4 species are new. Curiously, no fusulines were secured from the Leonard horizon, but the Word and Capitan equivalents are well represented with the dominant species identical with those of Texas and New Mexico. Even the striking new genus Rauserella has recently been discovered in Texas.

Some 36 species of brachiopods have been recognized and of these 10 species and 1 variety are new, but most of the rest are represented in the faunas of Texas and New Mexico, and the general aspect of the faunas indicates close affinity with the faunas of the Permian basin north of the Rio Grande. Due to the limited quantity of material available for study and the fragmentary character of much of it, many of the species are uncertainly identified.

Ammonites are represented by 21 genera and 34 species, of which 1 genus and 12 species and 1 variety are new. These indicate four well-defined faunal zones. Of these the lowest is the zone of Perrinites, which correlates with the Leonard horizon of Texas; the second is the zone of Waagenoceras, which correlates with the Word formation of Texas and the middle division of the Delaware Mountain sandstone; the third is the zone of Timorites, which is approximately of the age of the Capitan limestone and the upper division of the Delaware Mountain sandstone; the fourth, the zone of Kingoceras, is based upon a new genus and new species and may represent a horizon somewhat younger than the marine faunas of Texas and New Mexico. Kingoceras is associated with a new species of the fusuline genus Polydiexodina.

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