It has been attempted to give in this paper, for the first time, an idea of the tectonics of the Republic of Mexico as a whole, based on the concepts developed, mainly by alpine geologists, for the study of orogenic processes.

With this object in view, the paper was divided in two parts; the first, in which was established the meaning of the concepts in order to fix the sense in which they will be used and to point out in a general way the orogenic processes to which the Republic of Mexico was subjected. The second part in which are described the major structural units, from a tectonic point of view, especially as to the character and orientation of their folds, since the character of the major units is mainly established from the character of the folds.

There have been established in the foreland of the Mexican geosyncline, three shelves, a large massif, two small ones, and four basins. There have been postulated for the hinterland a great massif or continent, two large massifs, two medium, and four small massifs as well as three basins. The foredeep of the geosynclinal folds has been indicated in three regions, as established by the "flysch" sediments deposited therein.

It is considered that the geosynclinal folds of the Mexican Cordillera begin west of Ciudad Juarez toward Torreon, where they turn toward Monterey, and from there, southeast down to the Coatzacoalcos River, then northwest up to the Chalchijapa River and on reaching the narrowest part of the Isthmus of Tehuantepec they turn east-southeast into Guatemala.

It is established that the orogeny which gave rise to this folding started at the end of the Cretaceous in the northern part of the Republic and at the beginning of the Eocene in the rest of the country, and ended at the end of the Eocene except in the southeast where it lasted until the beginning

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of the Oligocene. Furthermore, there were previous and posthumous movements, the last of which is very important in the Isthmus and in the Macuspana and Pichucalco basins.

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