chap. tiii. Distribution of Gravel. 227 



force, a little onwards. We can thus understand how 

 oceanic or tidal currents of no great strength, or that 

 recoil movement of the bottom- water near the land, 

 called by sailors the ' 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. 1 

 The sea acts in another and distinct manner in 

 the distribution of pebbles, namely by the waves on the 

 beach. Mr. Palmer, 2 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. This agency must be powerful 

 in mingling together and disseminating pebbles derived 

 from different sources : we may, perhaps, thus under- 

 stand the wide distribution of the gallstone-yellow 

 porphyry ; and likewise, perhaps, the great difference in 

 the nature of the pebbles at the mouth of the Santa 



1 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 Tierradel 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 ' Voyage of the Adventure and Beagle,' vol. i. p. 375, and ' Appen- 

 dix,' p. 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. 



2 ' Philosophical Transactions,' 1834, p. 576. 



11 



