6oz THE POPULAR SCIENCE MONTHLY. 



basin of the Rhine from Basel down to the ridge beyond Mayence, 

 which has been subsequently cut through by the river into the pictu- 

 resque gorges between Bingen and the Siebengebirge. This lake was 

 filled up with red sand and mud, limestone, and beds of rock-salt. 

 Where the eastern Alps now rise, the inclosed water-basins were the 

 scene of a long-continued growth of dolomite, out of which in later 

 ages the famous dolomite mountains of the Tyrol were carved. 



These salt lakes of the Triassic period seem to have been every- 

 where quietly effaced by a wide-spread depression, which allowed the 

 water of the main ocean once .more to overspread the greater part of 

 Europe. This slow subsidence went on so long as to admit of the ac- 

 cumulation of masses of limestone, shale, and sandstone, several thou- 

 sand feet in thickness, and probably to bring most of the insular tracts 

 of Central Europe under water. To this period, termed by geologists 

 the Jurassic, we can trace back the origin Of a large part of the rock 

 now forming the surface of the continent, from the low plains of Cen- 

 tral England up to the crests of the northern Alps, while in the Medi- 

 terranean basin, rocks of the same age cover a large area of the pla- 

 teau of Spain, and form the central mass of the chain of the Apen- 

 nines. It is interesting to know that the northwest of Britain contin- 

 ued still to rise as land in spite of all the geographical changes which 

 had taken place to the south and east. We can trace even yet the 

 shores of the Jurassic sea along the skirts of the mountains of Skye 

 and Ross-shire. 



The next long era, termed the Cretaceous, was likewise more re- 

 markable for slow accumulation of rock under the sea than for the 

 formation of new land. During that time the Atlantic sent its waters 

 across the whole of Europe and into Asia. But they were probably 

 nowhere more than a few hundred feet deep over the site of our con- 

 tinent, even at their deepest part. Upon their bottom there gathered 

 a vast mass of calcareous mud, composed in great part of f oraminif era, 

 corals, echinoderms, and mollusks. Our English chalk which ranges 

 across the north of France, Belgium, Denmark, and the north of Ger- 

 many, represents a portion of the deposits of that sea-floor. Some of 

 the island spaces which had remained for a vast period above water, 

 and had by their degradation supplied materials for the sediment of 

 successive geological formations, now went down beneath the Creta- 

 ceous sea. The ancient high-grounds of Bohemia, the Alps, the Pyre- 

 nees, and the Spanish table-land, were either entirely submerged, or 

 at least had their area very considerably reduced. The submergence 

 likewise affected the northwest of Britain ; the western Highlands of 

 Scotland lay more than one thousand feet below their present level. 



When we turn to the succeeding geological period, that of the 

 Eocene, the proofs of wide-spread submergence are still more striking. 

 A large part of the Old World seems to have sunk down ; for we find 

 that one wide stretch of sea extended across the whole of Central 



