living reefs. But it should be observed that I had during the
two previous years been incessantly attending to the effects on
the shores of South America of the intermittent elevation of the
land, together with denudation and the deposition of sediment.
This necessarily led me to reflect much on the effects of
subsidence, and it was easy to replace in imagination the
continued deposition of sediment by the upward growth of corals.
To do this was to form my theory of the formation of barrier-
reefs and atolls.

Besides my work on coral-reefs, during my residence in London, I
read before the Geological Society papers on the Erratic Boulders
of South America ('Geolog. Soc. Proc.' iii. 1842.), on
Earthquakes ('Geolog. Trans. v. 1840.), and on the Formation by
the Agency of Earth-worms of Mould. ('Geolog. Soc. Proc. ii.
1838.) I also continued to superintend the publication of the
'Zoology of the Voyage of the "Beagle".' Nor did I ever intermit
collecting facts bearing on the origin of species; and I could
sometimes do this when I could do nothing else from illness.

In the summer of 1842 I was stronger than I had been for some
time, and took a little tour by myself in North Wales, for the
sake of observing the effects of the old glaciers which formerly
filled all the larger valleys. I published a short account of
what I saw in the 'Philosophical Magazine.' ('Philosophical
Magazine,' 1842.) This excursion interested me greatly, and it
was the last time I was ever strong enough to climb mountains or
to take long walks such as are necessary for geological work.

During the early part of our life in London, I was strong enough