918 PHYSIOGRAPHICAL GEOLOGY. [Book VII. 
must have taken place between the Lower Silurian and the Car- 
boniferous Limestone periods. If, in following the range along its 
course, we find the Carboniferous Limestone also highly inclined and 
covered unconformably by the Upper Coal-mneasures (¢ ¢), we should 
know that a second uplift of that portion of the ground had taken 
place between the time of the Limestone and that of the Upper 
Coal-measures. Moreover, as the Coal-measures were laid down 
below the sea-level, a third uplift has subsequently occurred whereby 
they were raised into dry land. By this simple and obvious kind 
of evidence the relative ages of different mountain chains may be 
compared. In most great mountain chains, however, the rocks have 
been so intensely crumpled, and even inverted, that much labour 
may be required before their true relations can be determined. 
The Alps offer an instructive example of a great mountain system. 
formed by repeated movements during a long succession of geolo- 
gical periods. The central portions of the chain consist of gneiss, 
schists, granite, and other crystalline rocks, partly referable to the 
Archean series, but some of which appear to be metamorphosed 
Paleozoic, Secondary, and even older Tertiary deposits. It would 
appear that the first outlines of the Alps were traced out even in 
Archean times, and that after submergence, and the deposit of 
Paleozoic formations along their flanks, if not over most of their 
site, they were re-elevated into land. From the relations of the 
Mesozoic rocks to each other, we may infer that several renewed 
uplifts after successive denudations took place before the beginning 
of Tertiary times; but without any general and extensive plication. 
A large part of the range was certainly submerged during the 
Eocene period under the waters of that wide sea which spread 
across the centre of the Old World, and in which the nummulitic 
limestone and flysch were deposited. But after that period the 
grand upheaval took place to which the present magnitude of the 
mountains is chiefly due. ‘The older Tertiary rocks, previously 
horizontal under the sea, were raised up into land, together with 
the older formations of the chain, and were crumpled, dislocated, and 
inverted. So intense was the compression to which the Hocene clays 
and sands were subjected that they were converted into hard and 
even somewhat crystalline rocks. It is strange to reflect that the 
enduring materials out of which so many of the mountains, cliffs, and 
pinnacles of the Alps have been formed are of no higher geological 
antiquity than the London clay and other soft Hocene deposits of 
the south of England. At a later stage of Tertiary time renewed 
disturbance led to the destruction of the lakes in which the molasse 
had accumulated, and their thick sediments were thrust up into 
large broken mountain masses, such as the Nighi, Rossberg, and 
other prominent heights along the northern flank of the Alps. 
Since that great movement no paroxysm seems to have affected the 
Alpine region except the earthquakes, which from time to time show 
the process of mountain-making to be only suspended or still slowly 
in progress. 

