A mass of Granitic, Cambrian, and Silurian rocks forms the central plateau of Spain, extending in a south-easterly direction from Galicia to the valley of the Guadalquiver, and spreading to the north-east, as shown by the chains of the Guadarrama and the mountains of Toledo, to terminate in the Celtiberian range, running nearly parallel to the Ebro by Soria and the Moncayo. In this mass the main folds of the strata appear to run in a south-easterly, the main fractures in a north-easterly, direction; whence the gridiron arrangement of the mountain chains and river valleys, directed by these leading features of the rocky structure. Great buttresses of the Carboniferous formation occupy the corners of the central mass, to the north and south-west, and occasional patches of its upper and coal-bearing beds are scattered over the interior. The whole valley of the Ebro occupies a trough of Secondary rocks, which extend in a south-easterly direction from the Bay of Biscay to the Mediterranean, forming a wide boundary to the older central mass, and running along the north coast towards Oviedo. The Secondary formations of the Ebro sweep over the chain of the Moncayo on to the central plateau by Burgos, Soria, and Calatayud; and their latest member--the Upper Cretaceous--advances in two long tongues on to the granite of the Guadarrama, and far to the east of Madrid, it being probable that at least this member formerly extended over the central plateau. Another wide band of Secondary rocks, running in a north-easterly direction, forms the long strip of Andalusia south of the Guadalquiver; and by Valencia and Cuenca this band is widely prolonged to the Ebro basin; otherwise, a narrow and interrupted strip along the south coast, and a bay-like expanse from the Atlantic, between Lisbon and Oporto, are the only Secondary tracts of the Peninsula. These Secondary rocks are however in great part concealed by Eocene Tertiary beds, formed in marine gulfs in the valley of the Ebro and the Guadalquiver, and overlaid by Eocene and Miocene fresh-water deposits; the latter being also represented by vast lacustrine sheets, which contemporaneously accumulated, and conceal the crystalline and palaeozoic formations in the elevated river basins of the central primary plateau. Patches of Pliocene sands and clays along the Mediterranean coast, sheets of diluvial gravels below the mountains, and alluvial sands along the larger rivers represent the local and most recent effects of water and ice.