Boor VII.| MATERIALS OF THE LAND. 911 
placement, afford an indication of its nature and amount. Having 
been laid down in wide sheets on the sea-bottom, they must have 
been originally, on the whole, level or at least only gently inclined. 
Any serious departure from this original position must therefore be 
the effect of displacement, so that stratification forms a kind of datum- 
line from which such effects may be measured. 
Further, it is not less apparent that the sedimentary formations, 
besides having suffered from disturbance of the crust, have under- 
gone extensive denudation. Hven in tracts where they remain 
horizontal, they have been carved into wide valleys. Their detached 
outliers stand out upon the plains as memorials of what has been 
removed. Where on the other hand they have been thrown into 
inclined positions, the truncation of their strata at the surface points 
to the same universal degradation. Here again the lines of stratifi- 
eation may be used as datum-lines to measure approximately the 
amount of rock which has been worn away. 
While, therefore, it is true that, taken as a whole, the dry land 
of the globe owes its existence to upheaval, it is not less true that 
its present contours are due mainly to erosion. These two antago- 
nistic forms of geological energy have been at work from the earliest 
_ times, and the existing: land with all its varied scenery is the result 
of their combined operation. Each has had its own characteristic 
task. Upheaval has, as it were, raised the rough block of marble, but 
erosion has carved that block into the graceful statue. 
The very rocks of which the land is built up bear witness to this 
intimate co-operation of hypogene and epigeneagency. ‘The younger 
stratified formations have been to a large extent derived from the 
- waste of the older, the same mineral ingredients being used over and 
over again. This could not have happened but for repeated uplitts 
whereby the sedimentary accumulations of the sea-fioor were brought 
within reach of the denuding agents. Moreover, the internal 
characters of these formations point unmistakably to deposition in 
comparatively shallow water. Their abundant intercalations of fine 
and coarse materials, their constant variety of mineral composition, 
their sun-cracks, ripple-marks, rain-pittings, and worm-tracks, their 
numerous unconformabilities and traces of terrestrial surfaces, together 
with the prevalent facies of their organic contents, combine to de- 
monstrate that the main mass of the sedimentary rocks of the earth’s 
erust was accumulated close to land, and that no trace of really 
abysmal deposits is to be found among them, Irom these considera- 
tions we are led up to the conclusion that the present continental 
areas must have been terrestrial regions of the earth’s surface froma 
remote gevlogical period. Subject to repeated oscillations, so that 
one tract after another has disappeared and reappeared from beneath 
the sea, the continents, though constantly varying in shape and size, 
have yet maintained their individuality. We may infer likewisc 
that the existing ocean basins have probably always been the great 
depressions of the earth’s surface. 
