210 
FOREIGN EQUIVALENTS. 
recently made in various parts of Holland. Next year another 
excursion was made by me, through France to Italy, mainly with 
the object of examining the marine Sub-Apennine deposits, and 
the lacustrine beds of the upper Yal d’Arno. The route followed 
was through the Hhone valley and the Riviera, for I was 
desirous of judging to what extent the Alps might have formed 
a barrier to the migration of species in Pliocene times, and 
consequently what amount of resemblance we might expect 
between the Pliocene land faunas north and south of the 
mountains. 
These journeys, though restricted to some only of the more 
important localities, have enabled me to form a better idea of 
the character of the Pliocene deposits of western Europe. In 
compiling the following notes I have taken advantage of the 
most recent work of the different national Surveys; but where 
the Surveys do not happen to be available, the account is 
either from personal observation or from the best Memoirs. 
Anyone acquainted with the difficulties met with in a small 
area in England will understand that the correlation here 
adopted is only provisional, although formed after a study of the 
best evidence at present to be obtained. The views of foreign 
observers hEive not in all cases been accepted as to the correla¬ 
tion, but where I have ventured to differ from them, it has been 
on the ground that the particular horizon dealt with happens to 
be better represented or more fossiliferous in England, or its 
stratigraphical relations are here more clearly shown. 
In dealing with foreign correlation it is necessary to divide the 
Pliocene deposits into two parallel series—the marine and the 
freshwater. The reason of this classification is obvious. A narrow 
isthmus may separa.te two quite different marine faunas, while the 
land and freshwater species are the same throughout the whole 
of the adjoining area. A strait, on the other hand, may prevent 
the mingling of different groups of land and freshwater forms, 
while allowing the marine to spread freely. It must obviously 
have made a great difference in the natural history and climate 
of the adjoining areas, whether there was a strait or an isthmus 
where we now find the Straits of Dover. We will therefore 
first deal with the marine strata and their fossils, and afterwards 
with the land-animals and plants, A similar division is adopted 
in the lists of fossils in the Appendix. 
In Belgium and Holland the Pliocene period is only repre¬ 
sented by marine deposits, which do not extend eastward into 
Germany, and are only traceable a short distance southward into 
France. Whether they ever extended across the Bas Boulon- 
nais towards Paris, is unknown, for the same line of disturbance 
which has affected the Weald, and has caused the Lenham Beds 
to end so abruptly, has cut off the Diestian group not far from 
Calais. The correlation of the Pliocene strata of Belgium with 
those of England, involves no great difficulty, and the subject has 
already been alluded to in previous Chapters. There seem to be 
