"undertow" (which I presume must extend out seaward as far as the BREAKING
waves impel the surface-water towards the beach), may gain the power during
storms of sifting and distributing pebbles even of considerable size, and
yet without so violently disturbing them as to injure the encrusting
corallines. (I may take this opportunity of remarking on a singular, but
very common character in the form of the bottom, in the creeks which deeply
penetrate the western shores of Tierra del Fuego; namely, that they are
almost invariably much shallower close to the open sea at their mouths than
inland. Thus, Cook, in entering Christmas Sound, first had soundings in
thirty-seven fathoms, then in fifty, then in sixty, and a little farther in
no bottom with 170 fathoms. The sealers are so familiar with this fact,
that they always look out for anchorage near the entrances of the creeks.
See, also, on this subject, the "Voyages of the 'Adventure' and 'Beagle'"
volume 1 page 375 and "Appendix" page 313. This Shoalness of the sea-
channels near their entrances probably results from the quantity of
sediment formed by the wear and tear of the outer rocks exposed to the full
force of the open sea. I have no doubt that many lakes, for instance in
Scotland, which are very deep within, and are separated from the sea
apparently only by a tract of detritus, were originally sea-channels with
banks of this nature near their mouths, which have since been upheaved.)

The sea acts in another and distinct manner in the distribution of pebbles,
namely by the waves on the beach. Mr. Palmer, in his excellent memoir on
this subject, has shown that vast masses of shingle travel with surprising
quickness along lines of coast, according to the direction with which the
waves break on the beach and that this is determined by the prevailing
direction of the winds. ("Philosophical Transactions" 1834 page 576.) This
agency must be powerful in mingling together and disseminating pebbles
derived from different sources: we may, perhaps, thus understand the wide
distribution of the gallstone-yellow porphyry; and likewise, perhaps, the