During the later Tertiary period all these desert regions would appear to have been covered by an Asian Mediterranean or, at all events, by vast fresh-water lakes, a conclusion which seems to be well warranted by the existence of salt-stained depressions of a lacustrine character; by traces of former lacustrine shore-lines, more or less parallel and concentric; by discoveries of vast quantities of fresh-water mollusc shells (e.g.
Deposits of the Tertiary period form the basis of more than half the state, extending from the border of the Cretaceous westward nearly to the Yazoo Delta and the Mississippi Bottom, and southward to within a few miles of the Gulf coast.
It is unfortunately still very difficult to correlate even approximately the strata on the two sides of the Atlantic, and there is great doubt as to what strata belong to each division of the Tertiary period even in different parts of North America.
During the Tertiary period the great volcanoes of the Andes were formed, and there were smaller eruptions in the Sierras.
Late in the Tertiary period vast sheets of lava poured from many points of the Great Dividing Range of eastern Australia.
During the Tertiary period arms of the sea spread into France in the Paris basin from the north, in the basins of the Loire and the Garonne from the west, and in the Rhne area from the south.