540 Geological Society* 



were much more numerous on the isthmus and its sides at the 

 height of 150 feet, than on any other part of the surrounding coun- 

 try ; and as the sea must have flowed over this isthmus in a channel, 

 previous to the amount of elevation, ascertained to have taken place 

 here within the post-pleiocene period, the position of these houlders 

 proves, according to Mr. Darwin, even more clearly than the cases 

 occurring in Tierra del Fuego, the evident relationship between 

 their distribution and the lines of anciently existing sea-channels. 

 In the southern half of Chiloe, and on one of the Chonos islands, the 

 author discovered a deposit of hardened mud, including far trans- 

 ported, angular and rounded fragments, and resembling the till of 

 the Straits of Magellan. In a layer of loose sand at the base of the 

 cliff in the latter locality, he noticed a quantity of comminuted ma- 

 rine shells with a fresh aspect ; and at Chiloe he also observed, at a 

 point where a mass of till passed into finely grained laminae, small 

 fragments of a Cytheraea. 



With respect to the age of the boulder formation of Chiloe, Mr. 

 Darwin offers no precise remark, but he says that it probably occurs 

 within the post-pleiocene period, because at a height of 350 feet on 

 the peninsula of Lacuy, and therefore considerably above the level 

 of this formation, a great bed of existing sea-shells was observed, 

 and neither the boulder nor accompanying beds appear to have been 

 of deep-water origin. Similar evidence was adduced respecting the 

 age of the till of Tierra del Fuego. North of 41° 47' S. lat., Mr. 

 Darwin did not observe on the Pacific side of South America either 

 boulders or till ; nor any north of the Straits of Magellan, on the 

 shores of the Atlantic side ; and he accounts for the absence of erra- 

 tic blocks in the latter region by its great distance from the Cor- 

 dillera. He is also strongly of opinion that till will be found to 

 be limited to the latitudes in which true boulders occur. 



Glaciers, %c. — In the concluding part of his memoir, the author 

 offers a few remarks on the glaciers of Tierra del Fuego, and on the 

 transport of the boulders. He did not disembark on any glacier, 

 but in the Beagle and Magdalen channels he passed within 2 miles 

 of several. The mountains were covered with snow, and the gla- 

 ciers formed many short arms, terminating at the beach in low per- 

 pendicular cliffs of ice. Their surface, to a considerable height on 

 the mountains, was perfectly clean and of a bright azure colour ; and 

 the former condition he ascribes to their shortness, to their not 

 being flanked by overhanging precipices, and to their not being 

 formed by the junction of two or more smaller streams. The de- 

 scent of the glaciers, Mr. Darwin states, cannot be very slow, as vast 

 masses continually break off with a great noise, and produce a tu- 

 multuous surf on the adjacent beaches. The glaciers in the Beagle 

 Channel were generally bordered by a tongue of land composed of 

 huge fragments of rock, and many boulders were strewed on the 

 neighbouring shores. The glacier which he approached most 

 closely descended to the head of a creek formed on one side by a 

 wall of mica-slate, and on the other by a broad promontory from 

 50 to 60 feet high, on which he landed : it appeared to consist en- 



