As to "epoch" and "period," I use them as synonyms to avoid repeating the
same word.

3. Rate of deposition and geological time. Here no doubt I may have gone
to an extreme, but my "28 million years" may be anything under 100
millions, as I state. There is an enormous difference between mean and
maximum denudation and deposition. In the case of the great faults the
upheaval along a given line would itself facilitate the denudation (whether
sub-aerial or marine) of the upheaved portion at a rate perhaps a hundred
times above the average, just as valleys have been denuded perhaps a
hundred times faster than plains and plateaux. So local subsidence might
itself lead to very rapid deposition. Suppose a portion of the Gulf of
Mexico, near the mouths of the Mississippi, were to subside for a few
thousand years, it might receive the greater portion of the sediment from
the whole Mississippi valley, and thus form strata at a very rapid rate.

4. You quote the Pampas thistles, etc., against my statement of the
importance of preoccupation. But I am referring especially to St. Helena,
and to plants naturally introduced from the adjacent continents. Surely if
a certain number of African plants reached the island, and became modified
into a complete adaptation to its climatic conditions, they would hardly be
expelled by other African plants arriving subsequently. They might be so,
conceivably, but it does not seem probable. The cases of the Pampas, New
Zealand, Tahiti, etc., are very different, where highly developed
aggressive plants have been artificially introduced. Under nature it is
these very aggressive species that would first reach any island in their
vicinity, and, being adapted to the island and colonising it thoroughly,
would then hold their own against other plants from the same country,
mostly less aggressive in character.