310 
IMPEEFECTION OF THE 
Chap. IX. 
subsidence, the great archipelagoes still areas of oscilla¬ 
tions of level, and the continents areas of elevation. 
But have we any right to assume that things have thus 
remained from the beginning of this world ? Our con¬ 
tinents seem to have been formed by a preponderance, 
during many oscillations of level, of the force of ele¬ 
vation ; but may not the areas of preponderant move¬ 
ment have changed in the lapse of ages? At a 
period immeasurably antecedent to the silurian epoch, 
continents may have existed where oceans are now 
spread out; and clear and open oceans may have 
existed where our continents now stand. Nor should 
we be justified in assuming that if, for instance, the 
bed of the Pacific Ocean were now converted into a 
continent, we should there find formations older than 
the Silurian strata, supposing such to have been for¬ 
merly deposited; for it might well happen that strata 
which had subsided some miles nearer to the centre of 
the earth, and which had been pressed on by an enor¬ 
mous weight of superincumbent water, might have 
undergone far more metamorphic action than strata 
which have always remained nearer to the surface. 
The immense areas in some parts of the world, for in¬ 
stance in South America, of bare metamorphic rocks, 
which must have been heated under great pressure, 
have always seemed to me to require some special 
explanation; and we may perhaps believe that we see 
in these large areas, the many formations long anterior 
to the Silurian epoch in a completely metamorphosed 
condition. 
The several difSculties here discussed, namely our 
not finding in the successive formations infinitely nu¬ 
merous transitional links between the many species 
which now exist or have existed; the sudden manner 
