430 Mr. Darwin on the Boulders 



It appears that masses of floating ice, by which fragments of rock are conveyed, 

 are produced in two ways, and under circumstances considerably different although 

 often acting together, namely, by the breaking off of icebergs from glaciers de- 

 scending into the sea, and by the actual freezing of the surface of the sea or its 

 tributary streams. Great boulders can be included in ice by this latter means 

 only (with rare exceptions) where the winter is extremely cold, as in the Gulf of 

 Bothnia and on the shores of North America. A large proportion of the fragments 

 thus enclosed will generally have been exposed to the wearing influences of the 

 sea-beach ; and from the ice being in a sheet, they will be hable to be repeatedly 

 stranded in shallow places, and thus to become still more worn. The other method 

 of transportal, namely, by the descent of glaciers to the sea-level, and the produc- 

 tion of icebergs, is far from necessarily requiring an extremely cold winter ; for the 

 low descent of glaciers seems to depend (other circumstances being alike) in a much 

 greater degree on the summer not being hot enough to melt the ice and snow, 

 than on the winter being very cold. Hence, as I have endeavoured to show in 

 my Journal (chap, xiii.), glaciers in South America descend to the sea from moun- 

 tains not very lofty, and in latitudes extraordinarily low compared with those in 

 Europe under which the same phaenomenon takes place ; and yet, the vegetable 

 and animal productions of this kind of climate have, in some degree, an inter-tro- 

 pical character. 



M. Agassiz has shown that blocks of rock are not imbedded in the ice of the 

 Swiss glaciers, except high up near their sources, and that those numerous masses 

 which lie on the surface, from not being exposed to much abrasion, remain angular : 

 hence only loose angular blocks of rock (as was the case with those on the floating 

 ice in Sir G. Eyre's Sound) can be transported by icebergs, detached from the gla- 

 ciers of temperate countries. And to effect this, the icebergs must be floated off 

 perpendicularly and in large masses, for otherwise the loose fragments would be 

 at once hurled into the sea. These remarks do not necessarily apply to icebergs 

 formed under a polar climate, for if a glacier in its descent, reached the sea before 

 the fragments of rock which had fallen on the soft snow had come to the surface, 

 icebergs would be produced with imbedded fragments of rock : I have described 

 in the ' Geographical Journal ' * the case of one huge fragment thus circum- 

 stanced, seen drifting far from land in the Antarctic Oceanf , 



As one of the above two methods of conveying erratic boulders, namely, that by 

 icebergs from glaciers, is now in action on the South American shores, we are na- 



* Geographical Journal, 1839, p. 528. 



t Dr. Merten's observed many fragments of rock imbedded only just above the level of the sea in the 

 lateraal wall of the glaciers at Spitzbergen, but he never saw any in the cliffs of ice facing the sea — 

 Edinburgh New Phil. Journal, 1841, pp. 173 and 176. 



