Until a few years ago it was believed that the Territory of Patagonia was of an extremely simple and uniform geological structure; it was supposed that from the Colorado to the Straits of Magellan, and from the Atlantic to the base of the foot-hills of the Andes, it was constituted exclusively by the marine Patagonian Formation, on the age of which the most varied opinions were expressed, although all agreed in referring it to the Tertiary series. This uniformity was interrupted only at intervals by great sheets of basalt which appeared sometimes at the surface, sometimes below the boulder-deposit.