426 Mr. Darwin on the Boulders 



evidently are similar to those called ' till,' and I feel nearly sure that they were un- 

 stratified ; but not being then aware of their interest, I merely compared them to 

 the deposits in eastern Tierra del Fuego, and I here record only what I at the time 

 wrote down. On the inner side of one of the Chonos Islands, a little south of 

 Chiloe, in lat. 43° 50', there are cliffs about 300 feet high, which I likewise only 

 partially described, as formed of blackish hardened mud with scattered pebbles of 

 various sizes, some well rounded, some but slightly : as I especially noted that the 

 gravel in the upper part of the cliif was stratified, I presume there was little ar- 

 rangement in the lower. In one layer of loose sand at the base of this cliff, where 

 the hardened mud passed into laminated sandy clay, I found a quantity of com- 

 minuted marine shells with a fresh aspect, but too much broken to be character- 

 ized. At Chiloe also, in one place where a mass of till passed into fine-grained, 

 laminated beds, I found two or three fragments of a Cytheraa. I must, however, 

 observe, that the absence of marine remains in these beds of till, is much less re- 

 markable than in those of Great Britain, because the surrounding stratified form- 

 ations here contain but very few shells. Indeed in Chiloe I found none, excepting 

 where the sandstone contained calcareous concretions. 



With respect to the age of the boulder formation of Chiloe, and I may add, 

 of Tierra del Fuego, I have no precise information. The whole east coast of 

 the island has been elevated certainly from twenty to forty feet, and probably much 

 more, within the post-pliocene period ; for on the peninsula of Lacuy (near where 

 I have described the boulders on the low neck) I found, at a height of 350 feet, a 

 great bed of existing shells, out of which forest trees were growing. I have stated, 

 that in eastern Tierra del Fuego the boulder formation has also been elevated 

 within this same period. Without we suppose that the boulders were deposited in a 

 deep sea, which does not appear very probable from the nature and stratification 

 of the accompanying deposits, or without we suppose great oscillations of level of 

 the land, the boulder formation must have been accumulated since the commence- 

 ment of the post-pliocene sera, or but little before it. From indirect evidence, 

 also, and therefore to be received with limited confidence, I came to the same con- 

 clusion with regard to the boulders on the lower plain at Santa Cruz. It is worthy 

 of notice, that geologists have referred the dispersal of the boulders which lie scat- 

 tered over the temperate parts of the northern hemisphere to this period. 



I have stated in my Journal, that northward of Chiloe (lat. 41° 47') I met with 

 no fragments of far-transported rock which can be classed with the erratic boulders 

 just described ; — their great size, frequent angularity, and complete separation by 

 wide valleys or arms of the sea from their parent source, being taken as their di- 

 stinctive characters : I had opportunities of observing them in the country around 

 Valdivia, Concepcion, and many parts of central and northern Chile. In these 



