66 



ERA OF THE TERTIARY FORMATION. — 

 MAMMALIA ABUNDANT. 



The chalk-beds are the highest which extend over a 

 considerable space ; but in hollows of these beds, compa 

 ratively 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. 

 There is a patch or fragment of the formation in one of 

 the Hebrides. 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 comparatively a local 

 formation, it is not of the less importance as a record of 

 the condition of the earth during a certain period. As in 

 other formations, it is marked, in the most distant locali- 

 ties, by identity of organic remains. 



The hollows filled by the tertiary formation must be 

 considered as the beds of estuaries 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 ; next, a second fresh-water forma- 

 tion, in which the material of the celebrated plaster of 

 Paris (gypsum) is included ; then, a second marine for- 

 mation of sandy and limy beds ; and finally, a third series 

 of fresh-wate 1 * strata. Such alternations occur in othei 

 examples of the tertiary formation likewise. 



The tertiary beds present all but an entirely new set oj 

 animals, and as we ascend in the series, we find more and 

 more of these identical with species still existing upon 

 earth, as if we had now reached the dawn of the present 

 itate of the zoology of our planet. By the study of the 

 shells alone, Mr. Lyell has been enabled to divide the 

 whole term into four sub-periods, to which he has given 



