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POPULAR SCIENCE REVIEW. 
Now while these marine and estuarine strata were being ac- 
cumulated in Hungary, Northern Germany, the Netherlands, 
Northern France, and the British Islands, a number of great 
lakes existed upon the land of the Oligocene period. These were 
gradually filled up, and obliterated by sedimentary deposits. 
Owing to the fortunate circumstance that, immediately after 
the close of the Oligocene period, a grand outburst of the vol- 
canic forces took place over very wide areas, these fresh-water 
deposits have been covered up and protected to some extent 
from destruction by denudation. It is thus that we find pre- 
served for our study the interesting lacustrine deposits of the 
Limagne, of Montbrison, of the Haute Loire, of Menat, and of 
many smaller lakes in the Auvergne, and of the lake-basins of 
Tepfitz, Falkenau, and Eger, in Bohemia. All of these lakes, 
with many others, of which all traces must have been removed 
by denudation, seem to have been in existence during the Oligo- 
cene period. 
If we now turn our attention to the districts immediately 
adjoining the great Alpine chains, we find the Oligocene form- 
ation represented by masses of strata of enormous thickness ; 
clays, sands, and conglomerates, accumulated sometimes in fresh- 
water lakes, at others in gulfs connected with the ocean. The 
Oligocene deposits of the Alpine district are estimated by the 
Swiss geologists as attaining a thickness of no less than from 
10,000 to 12,000 feet, and present a marked contrast in their 
physical character to the marine, estuarine, and lacustrine beds 
of the same age in other parts of Europe. But when we come 
to study the fossils of these Alpine formations, the parallelism 
of their several members with the divisions of the Upper, and 
Middle, and Lower Oligocene of Northern Europe becomes 
strikingly apparent. In the adjoining table an endeavour has 
been made to illustrate the correlation of the Oligocene and 
underlying deposits as exhibited in different parts of Western 
Europe (see p. 133). 
It is evident that the great and general subsidence which 
took place in the European area during the Eocene or Num- 
mulitic period had already come to a close, and had been 
succeeded by general elevatory movements before the com- 
mencement of the Oligocene. A great part of what is now 
Central Europe had become dry land, while an open sea stretched 
to the north and north-east of it. The Oligocene deposits 
were accumulated along the shores, and in the deeper waters 
of this sea, and in the estuaries of the great rivers which 
flowed into it from the south. At the same time great numbers 
of lakes existed on the surface of this Oligocene land, and into 
these were washed and thus preserved for our study many of 
the land animals and plants of the period. Before the close of 
