7?//)' 1 8, 1872] 



NATURE 



229 



PROFESSOR AGASSIZ'S SOUTH AMERICAN 



EXPEDITION* 



II. 



Tj-QR reasons which I will explain presently, I would mention 

 "'■ especially pebbles of a red porphyry, and others of a green 

 compact epidote, as common in the Port San Antonio foi mat ion. 

 The position of the Bay of San Mathias, its great length, and 

 more particularly the depression or denudation at Port San 

 .\ntonio, suggest the probability that the Rio Negro once 

 poured its waters into this large gulf instead of opening 

 directly into the ocean. I should add tliat while erratic pebbles 

 occur in such abundance at San Mathias Day, there are no hard 

 rocks in place upon which the peculiar marks of gLacial action 

 could be perceived. Nor would these stratified banks of pebbles, 

 even though unquestionably connected with the drift, afford in 

 themselves any unmistakable evidence of glacial derivation. 



As time and the circumstances of our vessel obliged me to re- 

 nounce the hope I had cherished of seeing at least the mouth 

 and the shore bluffs of the Rio Negro, .Santa Cruz, and Gallagos 

 Rivers, and also of visiting the F.alklands, I could not connect 

 my observations in San Mathias Bay with any other facts on the 

 eastern coast of Patagonia or its outlying islands. But after 

 rounding Cape Virgens we came into Possession Bay, where the 

 geology along the shore was of a most interesting character. All 

 along the noithern shores of the Straits of Magellan the tertiary 

 formation observed on the eastern shore of Patagonia is plainly 

 distinguishable even from a distance by its horizontal beds, 

 which are also visible upon the Fuegian coast. In Possession 

 Bay we landed to examine more closely the character of the 

 country, some of us with the intention of exploring more par- 

 ticularly the terraces above the shore bluffs ; while others were 

 bent upon a longer excursion to Mount Azmonand adjoining hills. 



About a mile from the shore bluff I found, nearly 150 ft. above 

 the sea level, a salt pool in which, to my great surprise, marine 

 shells identical with those now living along the shore were abun- 

 dant. They were in a perfect state of preservation, and many 

 of them were alive ; so that I gathered a number of specimens 

 with the living animal, whi h I have preserved in alcohol. The 

 most common were Mylihis, Biicciniiin^ Fissurella, Palrl/a, 

 Vohita, &c., all found in apparently the same numerical relation 

 as that in which they now exist in the sea below the cliff. The 

 presence of this pool with its living inhabitants shows a very 

 recent upheaval of the coast. The period at which it may ha\'e 

 taken place it is hardly possible to determine without a more 

 extensive siuvey. As the facts stand, it is a most valuable con- 

 firmation of Darwin's assertion of recent upheavals on this shore, 

 published more than thirty years ago ; though he attributes 

 phenomena to this cause, .and connects with it facts which h.id, 

 in my opinion, a different origin and another significance. At 

 the season of our visit to Possession B.ay, in March, when autumn 

 is approaching in this hemisphere, the pool was ne.arly dry, and 

 the little water left in it was intensely saline. Dr. While has 

 examined it chemically, and handed me the following report of 

 his .analysis : — " The specimen of water obtained from the pond 

 at Possession Bay was found to contain magnesia, lime, sulphuric 

 acid, chlorine, a small quantity of iron, and a trace of iodine. 

 It was about 2.I times as dense as ordin.iry sea water, as shown 

 both by hydrometric observation and by the total amount of 

 chlorine present ; organic matter in excess." The shores of the 

 pond showed plainly that in the rainy season it is three or four 

 feet deep, when no doubt the water is more like sea w.ater than 

 at the timeof our visit. From the innumerable tracks of guanacos. 

 It must be the const.ant resort of these .animals, and, indeed, 

 during the day we saw many of them moving in that direction. 

 A more palpable evidence of upheaval h.as not, as far as I know, 

 been observed before. Dead marine shells scattered over dry 

 land are not alwaj s conclusive evidence of the former presence 

 of the ocean, for they may have been dropped by birds or other 

 animals ; but a salt pond more than 100 ft. .above the sea level, 

 with the same shells alive as those now found on the shore, could 

 only be produced by an upheaval. The land beyond the first 

 shore bluff is horizontal. It ri.ses in regular terraces to about 

 400 ft. .above the level of the sea. This is also the general level 

 of the country, the surface of which is much ravined and 

 furrowed. I counted eight such terraces above the beach. They 

 all consist of tertiary deposits ; but upon the beach itself three 

 lower levels may be distinguished, their relative age being 

 marked by the presence or absence of vegetation on the sand. 

 • Reprinted from the New York Tribitnt. 



Upon the third terrace, a little above and more inward than the 

 salt pool, at the height of at least 150 ft. above the se.a, I found 

 a distinct moraine, in which the scratched pebbles were mingled 

 with the simply rounded ones in as large a proportion as in any 

 front moraine in actual contact with a glacier. This moraine 

 was arched, with the convexity turned northward, and the abrupt 

 slope southward, showing that the motive power which had 

 brought and left it there must have moved from the south in a 

 northerly direction. Higher up, to a level of .about 400 ft. above 

 tide water, there are also erratics scattered over the plain. At 

 the level of 400 ft. — the highest to which I ascended — I saw a 

 number of large, angular boulders. 



Here are facts, then, of great significance in close proximity, 

 namely, a pool containing marine shells alive, more than 100 ft. 

 above the level of the sea, showing a very recent rise of the tract 

 of land it occupied ; and an accumulation of pebbles and boulders 

 having all the characters of a glacial moraine, resting upon 

 one of numerous terraces, which seem to mark successive up- 

 heavals of the country. That these retreating levels only simu- 

 late the successive steps of a gradual upheaval, and are in fact 

 no evidence of such an occurrence, is proved by the geological 

 constitution of the ground, which is entirely made up of regular 

 tertiary beds, without a trace of shore pebbles. Darwin, who 

 has also observed the ]3henomena of subsidence and upheaval 

 characteristic of this region, was led to believe that the drift 

 was scattered over Patagonia by icebergs while the country was 

 submerged. The moraine upon one of these terraces mentioned 

 above shows, however, that the upheaval must have taken place 

 before the dispersion of the ilrift, and not .after. I say nothing 

 here of Pourtales's very interesting discovery of an extensive 

 field of extinct volcanoes to the north of Possession Bay, of 

 which Mount Azmon is the largest, since he has already sent 

 you an official report on the subject. His observations are 

 among the most valuable results of our geological work. The 

 suggestion presents itself at once that the upheaval of the region 

 may be connected with the former activity of those volcanoes. 

 Throughout the eastern p.art of the Straits of Magellan the shores 

 exhibited tertiary formations such as we had traced along the 

 Atlantic coast of Patagonia and in San Matthias Bay. Of 

 course in deposits of this kind we could not expect to find any 

 trace of smooth polished rocks. 



The last localities of recent geological age which we examined 

 carefully were Elizabeth and Magellan Islands. The latter is 

 almost entirely made up of glacial drift, among which there are 

 a good many large and small boulders with very smooth surfaces 

 and characteristic scratches, and some of them are of the same 

 red porphyry and epidote mentioned before. At Sandy Point 

 large accumulations of boulders are scattered over the whole 

 country, and the streets of the settlement are paved with them. 

 They are easily observed in their natural position on the banks 

 of the river, and in the cuts of the railroad leading to the coal 

 mines. Here the drift is auriferous. Seilor Viel, governor of 

 the colony, gave me very fine specimens of the gold collected in 

 the immediate neighbourhood of the settlement. I was also 

 much interested in the coal deposit. There are two considerable 

 seams of coal, one 6 ft. 6 in. thick, and another, So ft. higher 

 up, 3 ft. thick. The few species of fossils which I obtained there 

 in great quantity left upon me the impression that the coal is not 

 tertiary, liut belongs to the cretaceous formation. The most 

 characteristic of these fossils is an oyster, of the type of the 

 Ostna ddloidca, forming beds many feet in thickness. 



After passing Sandy Point the country assumes a completely 

 different aspect. The mountains rise to great heights on both 

 sides of the channel, in consequence of which the region may be 

 compared to the Alps, even though the loftiest peaks, such as 

 Mount Sarmiento, Mount Darwin, Mount Buckland, Mount 

 Barney, only measure from 6,000ft. to 7,000ft. But as their 

 base is washed by the ocean, and their slope is very steep, they 

 appear much higher than they really are. 



The neighbourhood of Sandy Point will ever be especially 

 interesting to Swiss geologists, from the fact that it recalls many 

 familiar scenes. Pourtales and I greatly enjoyed the comparison 

 with home scenery. In his work on the " Rocks of the Two 

 Hemispheres," and in his "Kosmos," Ilumbolct repeatedly 

 alludes to the striking similarity of the features exhibited by the 

 inorganic world in regions very distant from each other, and I 

 only follow in his footsteps if I say that Sandy Point and the 

 tracks north of it recalled to me the Jura and the more level 

 country at its foot, while the higher ranges to the south reminded 

 me of the Alps. The comparison might be carried into detail 

 without exaggeration. The first chain in sight from the channel, 



