420 Mr. Darwin on the Boulders 



all those who have to double it ; hence they probably set in a similar direction 

 through the above ancient channels, when more open and less crooked than the 

 Strait of Magellan now is. It is in accordance with this circumstance, that, in the 

 districts just described, and in that which we are immediately to treat of, the 

 boulders and smaller fragments have all travelled from mountains situated to the 

 west. 



In a space about forty miles broad at the extreme south-eastern part of Tierra 

 del Fuego, including Navarin and several smaller islands, the shores are fringed at 

 about an equal height, by a deposit very closely resembling the unstratified beds in 

 the Strait of Magellan. On the south side of Navarin Island it forms a small plain 

 (the only level land in that part of the country), fronted by a line of cliff several 

 miles in length, and about sixty feet in height. In this cliff there is not a trace of 

 stratification ; and the earthy, rather argillaceous mass contains fragments, some 

 angular, but mostly rounded, of all sizes, from mere particles to great boulders, 

 of nearly the same composition as the fragments in the Strait of Magellan. Similar 

 rocks do not occur in situ within sixty miles ; and probably some exist only at a 

 considerably greater distance. Within the eastern mouth of the Beagle channel, 

 forming part of the above-mentioned area, the cliffs are higher, and the beds are 

 sometimes regularly interstratified with layers of shingle. I cannot more accu- 

 rately describe the appearance of the cliffs around Navarin Island, than by the re- 

 mark which, at the time, I entered in my note-book, " that a vast debacle appeared 

 to have been suddenly arrested in its course." But this explanation always ap- 

 peared to me, from the width and openness of the channels both to the east and 

 west, and from the proofs of the very gradual elevation of the land in the neigh- 

 bouring countries, to be encumbered with the greatest difficulty. Hence the origin 

 of these beds, as well as of those in the Strait of Magellan, which, although unstra- 

 tified, are of submarine formation, remained quite inexplicable to me. This deposit 

 resembles the till of Scotland, the boulder formation of Northern Europe and of 

 the eastern coast of England, in the following respects, which clearly indicate, 

 as Mr. Lyell* has remarked, some peculiar origin ; first, in the entire absence of 

 stratification in one part of a bed, which in another shows, either throughout its 

 whole thickness or in alternate beds, signs of regular deposition; secondly, in the 

 close juxtaposition of fragments of far-transported rocks, varying in size from 

 minute pebbles to boulders, some being rounded and others angular ; and lastly, 

 as I believe, in the frequent occurrence of a capping of gravel. Mr. Lyell, after 

 having examined this kind of deposit (which for briefness I will call by the Scotch 

 term ' till ') in the several countries above specified, ascribes its origin to the 

 deposition, in a tranquil sea, of mud, sand and blocks, from melting drift-ice ; 



* Mr. Lyell on the Boulder Formation of East Norfolk. Philosophical Magazine, 1840, p. S^S. 



