352 Mr Darwin on the Ancient Glaciers oj Caeman'onshire, 



glacier, indicating the succession of waves of more or less com- 

 pact ice. In all the glaciers where I have yet distinctly ob- 

 served them, they appear to follow a regulated order of dis- 

 tances, nearly the same for a considerable space, but closer the 

 farther we ascend the glacier. I cannot help thinking that 

 they are the true annular rings of the glacier, which mark its 

 age, like those of a tree, only increasing instead of diminishhig 

 in breadth as the ice grows older, coinciding again with the 

 fact which I formerly established, that the higher part of a 

 glacier moves, generally speaking, more slowly than its lower 

 exti'emity. The different states of the glacier at different 

 seasons, the presence or absence of snow, or even the simple 

 difference of velocity at different seasons, would be sufficient 

 to account for this alternation of structure. There is no cause 

 so likely to produce it as some awiual change. I may add, 

 that some observations which I have already made on the dis- 

 tances of these bands, as well as information which I have en- 

 deavoured to collect, lead me at least to have some doubt as 

 to the correctness of the opinion generally entertained that 

 the glaciers are stationary in winter, perhaps even, that there 

 is any very great inequality in their march at different times 

 of the year. I am, my dear Sir, yours very truly, 



James D. Forbes. 

 Professor Jameson. 



Notes on the Effects produced by the Ancient Glaciers of Caer- 

 narvonshirey and on the Boulders transported by Floating 

 Ice* By Charles Darwin, Esq., M.A., F.R.S. and F.G.S. 



Guided and taught by the abstract of Dr Buckland's memoir 

 '* On Diluvio-Glacial Phenomena in Snowdonia and the ad- 

 jacent parts of North Wale3,"t I visited several of the loca- 

 lities there noticed ; and having familiarized myself with some 



* From the Philosophical Magazine, vol. xxi. No. 137. 

 t Read before the Geological Society, 15th December 1841 ; andtheabr 

 stract is publishei in the Atheneeum, 1842, p. 42, .; 



