74 



ERA OF THE TERTIARY FORMATION- 

 MAMMALIA ABUNDANT. 



THE chalk-beds are the highest which extend over a consi- 

 derable space; but in hollows of these beds, comparatively 

 limited in extent, there have been formed series of strata 

 clays, limestones, marls, alternating to which the name of 

 the Tertiary Formation has been applied. London and Paris 

 alike rest on basins of this formation, and another such basin 

 extends from near Winchester, under Southampton, and re- 

 appears in the Isle of Wight. A stripe of it extends along 

 the east coast of North America, from Massachusetts to 

 Florida. It is also found in Sicily and Italy, insensibly 

 blended with formations still in progress. Though compara- 

 tively a local formation, it is not of the less importance as a 

 record of the condition of the earth during a certain period. 



The hollows filled by the tertiary formation must be consi- 

 dered as the beds of estuaries and gulfs, left at the conclusion 

 of the cretaceous period. We have seen that an estuary, 

 either by the drifting up of its mouth, or a change of level in 

 that quarter, may be supposed to have become an inland sheet 

 of water, and that, by another change of the reverse kind, it 

 may be supposed to have become an estuary again. Such 

 changes the Paris basin appears to have undergone oftener 

 than once, for, first, we have there a fresh- water formation of 

 clay and limestone beds ; then, a marine-limestone formation ; 



