The Province of Huelva, famed for its mines from the times of the Ancients, lies in the south-western portion of Spain, in the upland between the basins of the Guadalquivir and Guadiana. To the north are the serrated ridges of the Sierra Morena and to the south the fertile plains of Huelva and Sevilla. This upland, now in a mature stage of erosion, forms the old coastal plain. It is composed of Palaeozoic rocks, chiefly Carboniferous, which lie unconformably on the pre-Cambrian gneisses and schists of the Sierra Morena. Miocene limestone and Pleistocene Drift in places obscure the Palaeozoic rocks. The Carboniferous rocks are principally slates and grits, the former being by far the more abundant. In what is usually regarded as Hercynian times and following great tectonic movements, the Carboniferous rocks were invaded by granites, porphyries, diorites, porphyrites, and diabase. The mineralization was associated with these intrusives and principally with the porphyries. Finally the area was reduced to a peneplain by erosion.