The valleys of Aragua form a narrow basin between granitic and calcareous mountains of unequal height. On the north, they are separated by the Sierra Mariara from the sea-coast; and towards the south, the chain of Guacimo and Yusma serves them as a rampart against the heated air of the steppes. Groups of hills, high enough to determine the course of the waters, close this basin on the east and west like transverse dykes. We find these hills between the Tuy and La Victoria, as well as on the road from Valencia to Nirgua, and at the mountains of Torito. * (* The lofty mountains of Los Teques, where the Tuy takes its source, may be looked upon as the eastern boundary of the valleys of Aragua.