THE VOYAGE OP THE BEAGLE 185 



mulated in bays, here along hundreds of miles of coast we 

 have one great deposit, including many tertiary shells, all 

 apparently extinct. The most common shell is a massive 

 gigantic oyster, sometimes even a foot in diameter. These 

 beds are covered by others of a peculiar soft white stone, in- 

 cluding much gypsum, and resembling chalk, but really of 

 a pumiceous nature. It is highly remarkable, from being 

 composed, to at least one-tenth of its bulk, of Infusoria. 

 Professor Ehrenberg has already ascertained in it thirty 

 oceanic forms. This bed extends for 500 miles along the coast, 

 and probably for a considerably greater distance. At Port 

 St. Julian its thickness is more than 800 feet ! These white 

 beds are everywhere capped by a mass of gravel, forming 

 probably one of the largest beds of shingle in the world: it 

 certainly extends from near the Rio Colorado to between 600 

 and 700 nautical miles southward; at Santa Cruz (a river a 

 little south of St. Julian), it reaches to the foot of the Cor- 

 dillera ; half way up the river, its thickness is more than 200 

 feet; it probably everywhere extends to this great chain, 

 whence the well-rounded pebbles of porphyry have been 

 derived: we may consider its average breadth as 200 miles, 

 and its average thickness as about 50 feet. If this great bed 

 of pebbles, without including the mud necessarily derived 

 from their attrition, was piled into a mound, it would form a 

 great mountain chain ! When we consider that all these 

 pebbles, countless as the grains of sand in the desert, have 

 been derived from the slow falling of masses of rock on the 

 old coast-lines and banks of rivers ; and that these fragments 

 have been dashed into smaller pieces, and that each of them 

 has since been slowly rolled, rounded, and far transported, 

 the mind is stupefied in thinking over the long, absolutely 

 necessary, lapse of years. Yet all this gravel has been trans- 

 ported, and probably rounded, subsequently to the deposition 

 of the white beds, and long subsequently to the underlying 

 beds with the tertiary shells. 



Everything in this southern continent has been effected 

 on a grand scale : the land, from the Rio Plata to Tierra del 

 Fuego, a distance of 1200 miles, has been raised in mass (and 

 in Patagonia to a height of between 300 and 400 feet), within 

 the period of the now existing sea-shells. The old and 



