EOCENE AREAS OF NORTH OF EUROPE. 
251 
cene those Tertiary strata which have been described in the 
last chapter as Lower Miocene, and to which M. Beyrich has 
given the name of Oligocene. None of these occur in the 
London Basin, and they occupy in that of Hampshire, as we 
have seen at p. 244, too insignificant a superficial area to be 
noticed in a map on this scale. They fill a larger space in 
the Paris Basin between the Seine and the Loire, and con¬ 
stitute also part of the northern limits of the area of the 
Netherlands which are shaded in the map. 
It is in the northern part of the Isle of Wight that we 
have the uppermost beds of the true Eocene best exhibited 
—namely, those which correspond in their fossils with the 
celebrated gypsum of the Paris basin before alluded to, p. 
231 (see Table, p. 252). That gypsum has been selected by 
almost all Continental geologists as affording the best line 
of demarkation between the Middle and Lower Tertiary, or, 
in other words, between the Lowe*r Miocene and Eocene for¬ 
mations. 
In reference to the annexed table I may observe, that the 
correlation of the French and English subdivisions here laid 
down is often a matter of great doubt and difficulty, not¬ 
withstanding their geographical proximity. This arises fro*m 
various circumstances, partly from the former prevalence of 
marine conditions in one basin simultaneously with fluviatile 
or lacustrine in the other, and sometimes from the existence 
of land in one area causing a break or absence of all records 
during a period when deposits may have been in progress 
in the other basin. As bearing on this subject, it may be 
stated that w^e have unquestionable evidence of oscillations 
of level shown by the superposition of salt or brackish-water 
strata to fluviatile beds; and those of deep-sea origin to 
strata formed in shallow water. Even if the upward and 
downward movements were uniform in amount and direc¬ 
tion, which is very improbable, their efiect in producing the 
conversion of sea into land or land into sea would be differ¬ 
ent, according to the previous shape and varying elevation 
of the land and bottom of the sea. Lastly, denudation, ma¬ 
rine and subaerial, has frequently caused the absence of de¬ 
posits in one basin of corresponding age to those in the oth¬ 
er, and this destructive agency has been more than ordina¬ 
rily effective on account of the loose and unconsolidated na¬ 
ture of the sands and clays. 
