418 Mr. Darwin on the Boulders 



nian formation, fringed by deposits of much more recent origin, the height of 

 which varies from about 100 to 250 feet. These lower, irregular plains have been 

 elevated within the post-pliocene period. They consist of fine-grained, earthy or 

 argillaceous sandstone, in very thin, horizontal, but sometimes inclined laminae, 

 and often associated with curved layers of gravel. On the borders, however, of the 

 eastern parts of the Strait of Magellan, this fine-grained formation often passes into, 

 and alternates with, great unstratified beds, either of an earthy consistence and 

 whitish colour, or of a dark colour and of a consistence like hardened coarse-grained 

 mud, with the particles not separated according to their size. These beds contain 

 angular and rounded fragments of various kinds of rock, together with great boul- 

 ders. At Elizabeth Island, within the Strait, there are good sections of this de- 

 posit in cliffs 150 feet high, and composed chiefly of whitish earth, with frag- 

 ments of syenite, greenstone, feldspathic rocks, clay and hornblendic slates and 

 quartz, most of which do not occur, in situ, in the neighbourhood. These frag- 

 ments are generally arranged without the slightest trace of order, — large and small, 

 angular and rounded being close together ; but in some parts of the cliff", the mass 

 is divided by beds of stratified shingle, and these are most frequent in the upper 

 part, — a fact which I observed in other places- 

 Few of the fragments much exceed in size a man's head, but there are nume- 

 rous large boulders on the beach. In the cliff" at Cape Negro, which is close to Eliza- 

 beth Island, and is of the same height and of nearly the same nature, I saw a great 

 boulder imbedded. This deposit at Nuestra Sehora de Gracia is rather finer grained, 

 and contains fewer fragments ; some of which are perfectly rounded, some quite 

 angular ; and a single one, of considerable size, is often imbedded by itself in 

 fine-grained and fine-laminated matter. I here, also, observed a boulder at least 

 four feet in diameter, projecting from the face of the cliff". In a neighbouring 

 cliff", a whitish mass fills up hollows in an underlying finer-grained bed. North 

 of Cape Virgins, close outside the mouth of the Strait, the chff"s are between 200 

 and 300 feet in height ; and they consist of an argillaceous sandstone in horizontal 

 laminae, as fine as roofing-slate, which in several places is interstratified with two 

 or three beds of the coarse nature just described, each stratum being from five feet 

 to twenty thick. These beds often thin out and become curvilinear at each end. The 

 imbedded fragments are of the same nature and shape as before mentioned ; and 

 their parent rock cannot be less, and probably is considerably more, than 120 geo- 

 graphical miles distant. In the other cases above described, the distance must be 

 at least sixty miles. The mountains, from which they all probably came, he west 

 and south-west. 



The numerous boulders before noticed on the beach at the foot of the cliff's on 

 Elizabeth Island, consist of the same varieties as the smaller imbedded fragments, 



