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The 1:5,000,000 map of Uruguay is a fairly good generalized representation of the geology of the country. The details of the contacts, however, are somewhat oversimplified.

The geology of Uruguay is well known in all major features, and some areas have been worked out in detail. Mention should be made of McMillan’s and Walther’s studies of the Precambrian, Falconer’s work on the Upper Paleozoic, Mendez Alzola’s description of the Lower Devonian fossils, and Lambert’s summary of the geology of the country, published in 1941. The following description is based on a perusal of all the available literature supplemented by the writer’s own field investigations in Uruguay.



The oldest rocks of Uruguay are exposed in the southern part of the country, especially along the coast of the Rio de la Plata between Colonia and Punta del Este. A narrow band of early Precambrian rocks is also known in the northeastern part of the country, between Tacuarembó and Aceguá. The predominant rocks are granitoid, augen, and foliated gneisses, but mica schists are also frequent. Conglomeratic gneisses have been reported from Punta del Este. Pegmatite and aplite dikes are numerous, and orthoamphibolites are conspicuous in the Cerro de Montevideo and the vicinity of Colonia.

The metamorphic rocks are highly folded and contorted; the schistosity is subvertical and parallel to the Atlantic Coast with a dominant northeast to north-northeast strike. Local deviations, however, are frequent.

In the department of Maldonado, McMillan has distinguished two generations of early Precambrian gneisses;

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