Geologists of the Third Asiatic Expedition of the American Museum of Natural History, collected specimens of the igneous rocks from the Gobi Desert of Central Mongolia. These represent the range of age and type in intrusives of the region. Approximately 175 satisfactory specimens were available for study. All were used, but detailed study was concentrated on fifty whose relative ages were definitely known. These show range in composition during each of several periods of important activity. They come chiefly from a zone extending southwest from the territory between Mt. Tuerin and Urga, on the northeast, to Artsa Bogdo and the Gurbun Saikhan, at the southwest (Fig. 1).

Field studies indicate four distinct age-groups of igneous rocks and two additional less definite ones in the western part, between Tsetsenwan and the Gurbun Saikhan. These groups are the products of distinct periods of igneous activity, separated by quiescent stages. . . .

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