In the upper parts of the valleys a number of lakes occur, occupying hollows and rock basins in the agglomerates and ashes, fed by springs, and feeding many of the streams that drain the mountain slopes.
But by far the most important of the Tertiary rocks are the volcanic lavas, agglomerates and ashes, which cover so much of the country.
Basic lavas, with andesites, trachytes, tuffs and agglomerates are the most common Scottish rocks of this period.
It is only in the more northerly part of the country that the pipes are filled with blue ground (or " kimberlite "), and that they are diamantiferous; but over a great part of Cape Colony have been discovered what are probably similar pipes filled with agglomerates, breccias and tuffs, and some with basic lavas; one, in particular, in the Riversdale Division near the southern coast, being occupied by a melilite-basalt.
Farther north nearly the whole of the depression is filled with lavas, tuffs and agglomerates, derived from the Tertiary and recent volcanoes which form the most striking feature of the Andes of Ecuador.
On the east side in North Glen Sannox Burn, they are associated with cherts, grits and dark schists with pillowy lavas, tuffs and agglomerates which, on lithological grounds, have been regarded as probably of the same age as the Arenig cherts and volcanic rocks in the south of Scotland.
In Ecuador the depression between the Eastern and Western Cordilleras is almost entirely filled with modern lavas and agglomerates; in Colombia the corresponding Cauca depression is almost free from such deposits.
In Minahassa, at the northern extremity, there is a large area of tuffs and agglomerates consisting chiefly of augite andesite, and in this area there are many recent volcanic cones.