Boulders and Unstratified Deposits of South America. 539 



ving that floating ice, charged with foreign matter, has been the chief 

 agent in its formation ; but he adds that it is difficult to understand 

 how the finest sediment was arranged in horizontal laminae, and 

 coarse shingle in beds, while stratification is totally, and often sud- 

 denly, wanting in the closely neighbouring till, if it be supposed 

 that the materials were merely dropped from melting drift ice ; and 

 he is disposed to think that the absence of stratification, as well as 

 the curious contortions described in some of the stratified masses, 

 are mainly due to the disturbing action of icebergs when grounded. 

 He believes also that the total absence of organic remains in these 

 deposits may be accounted for by the ploughing up of the bottom 

 by stranded icebergs, and the impossibility of any animal existing on 

 a soft bed of mud or stones under such circumstances. In confirm- 

 ation of the disturbing action of icebergs, Mr. Darwin refers to 

 Wrangell's remarks on their effects off the coast of Siberia. 



Chiloe. — North of latitude 47° and between it and the southern ex- 

 tremity of Chiloe, the author landed at several points, but saw no 

 boulders; and he explains their absence by the coast being at a distance 

 from the Cordillera, and separated from it by intervening high land. 

 At Chiloe erratic boulders, often of great size and consisting of granite 

 and sienite, occur in vast numbers along the whole line of the east- 

 ern and northern beaches, as well as on the islets parallel to the 

 eastern coast, and on the land at the height of upwards of 200 feet ; 

 but the author did not observe any on the western coast at the two 

 points which he examined, nor during an excursion of 30 miles 

 across the high central portion of the island. Chiloe consists, as far 

 as Mr. Darwin ascertained, of mica-slate and volcanic formations, 

 extensively bordered, but chiefly on the eastern and northern sides, 

 by a horizontally-bedded tertiary sandstone and volcanic grit. On 

 the eastern coast, the land is indistinctly modelled into successively 

 rising plains, the surfaces of the upper and the whole thickness of 

 some of the lower being in general composed of stratified shingle. 

 A few boulders occur in this gravel ; and as the shores have been 

 extensively denudated, Mr. Darwin infers that most of the very 

 numerous blocks on the beaches were originally included in it. At 

 the northern end of the island, the granitic and sienitic boulders are 

 intermingled, but 30 miles to the southward, the author noticed only 

 granite blocks. The parent rock he believes lies in the Cordillera ; 

 and several of the varieties of granite and sienite at the northern 

 end of the island are stated, on the authority of an intelligent resi- 

 dent, to form whole mountains in Reloncavi Sound, on the opposite 

 part of the main land. The larger masses were quite angular, and 

 resembled fragments at the foot of a mountain. One block measured 

 15 feet in length, 11 in breadth, and 9 in height ; another, of a pen- 

 tagonal form, 11 feet on each side, and at one part projected 16 feet 

 above the sand, in which it was partly buried. 



At the extreme northern point of Chiloe, a headland 250 feet 

 high is joined to Lacuy peninsula by a low neck of land ; and from 

 its composition, height and stratification, Mr. Darwin ascertained 

 that it was once continuous with the opposite coast. The boulders 



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