ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxi 



pear to have been going on in a much earlier period, for he observed 

 extensive beds of sediment undistinguishable from the Pampas deposit 

 underlying old tertiary rocks on which the true P-ampean formation 

 rests. He further supposes, that the bottom of that sea and estuary 

 was gradually rising during the slow progress of the deposition, and 

 that the animals, whose remains are buried, lived on the adjoining land, 

 and that when either dying a natural death, or drowned by inundations, 

 their bodies were floated off to sink in the mud, and be entombed near 

 the spots where they had lived ; for not only are entire skeletons found, 

 but even when the bones are separated, they are often met with lying in 

 their proper relative positions, and they never bear the marks of having 

 been worn by attrition in a transport by floods from a distant region. 



With regard to the age of the Pampean formation, it appears from 

 the uniformity in its composition, the specific identity of the mammi- 

 ferous remains over its vast area, and their occurrence throughout 

 its whole depth, that it belongs to one geological epoch ; and that 

 from its association with shells now living in the adjoining sea, from 

 the many proofs that the bodies of the animals were imbedded in a 

 fresh state, and that they therefore had co-existed with the shells, it 

 must be of the pleistocene sera. " I feel little doubt," says Mr. 

 Darwin, " that the extinction of the large quadrupeds did not take 

 place until the time when the sea was peopled with all, or nearly all, 

 its present inhabitants*." 



From the southern termination of the Pampean deposit at the 

 Rio Colorado, another vast area of detrital matter, very different in 

 its nature, but chiefly of the same age, commences ; for nearly the 

 whole of Patagonia is covered with gravel, capped by a thin irregular 

 bed of sandy earth, and it extends across the Straits of Magellan 

 into Tierra del Fuego. Near the coast it is generally from 10 to 30 

 feet in thickness, but at a distance of 110 miles inland it has a depth 

 of 212 feet, and Mr. Darwin is of opinion that its average depth may 

 be not less than 50 feet. It covers an area in Patagonia of 630 by 

 200 miles, rising from the coast to the foot of the Cordillera, a 

 height of between 3200 and 3300 feet. Porphyries of different kinds 

 constitute the chief mass, but there are also pebbles of other crystal- 

 line felspathic rocks, basalts, compact clay-slate and quartzose schists, 

 all derived from the mountainous country on the west, and from the 

 basaltic dykes or streams that occur in different parts of the inclined 

 plane near these mountains. The absence of angular fragments, and 

 the perfectly rounded condition of the pebbles, indicate long-con- 

 tinued attrition. The rarity and inconsiderable size of the streams in 

 Patagonia make the transport and wearing by river-action improbable ; 

 "moreover," the author adds, "in the case of the one great and 

 rapid river of Santa Cruz, we have good evidence that its transporting 

 power is very trifling. This river is from 200 to 300 vards in width, 

 about 17 feet deep in its middle, and runs with a singular degree of 

 uniformity five knots an hour, with no lakes, and scarcely any still 

 reaches : nevertheless, to give one instance of its small transporting 



* Introduction to Professor Owen's * Description of the Fossil Mammalia col- 

 lected by Mr. Darwin.' 



