Mr. O. Fisher, Clk. — Glacial Erosion of Lake Basins. 253 



appeared always contained evidence of advance in development over 

 those which had previously reached the areas. 



In the Cambrian and Lower Silurian deposits, the lines of migra- 

 tions of the faunas are easily followed out, for they advanced generally 

 along areas of deepest submergence only; and even after most of 

 the land had been completely submerged, the tendency in the faunas 

 to follow the former lines is still shown in the appearance of the 

 higher forms of marine life (Vertebrates) in earlier rocks over the 

 western areas of Europe than in the more eastern areas. 



The Devonian Epoch is the first stage in the succession which 

 gives decided proof of how far life on the land had j^rogressed con- 

 temporaneously with marine life. The advance made in land life 

 does not seem to have been much behind that in the sea, and it is 

 evident that it had been going on gradually on the portions of the 

 old land remaining above the water, and that it had attained this 

 state by successive steps in the long ages indicated by the Laurentian, 

 Cambrian and Silurian periods. At the time the pre-Cambrian 

 continent commenced to subside, the climate was probably too severe 

 to allow vegetation to have progressed to any great extent on lands 

 in the higher latitudes; but as the land became depressed and the 

 climate milder, the portions still above water became clothed with 

 abundant vegetation, and inhabited by the lower forms of terrestrial 

 life. All these facts tend to show that life on land and in the sea 

 progressed chiefly in warm areas, and that the northern latitudes 

 were not on the whole favourable in the earliest time to this progress. 

 This is the explanation to my mind why the record is not more 

 perfectly preserved to us in these northern regions, which, from 

 these very circumstances of repeated change, is almost the only one 

 now open to our examination. 



IV. — On the Theory of the Erosion of Lake-Basins by Glaciers. 

 By Mr. 0. Fisher, Clk., M.A., F.G.S. 



IT is now more than eleven years since I ventured to take a part in 

 the discussion then raised by Professor Ramsay having lately 

 published his celebrated theory of the Erosion of Lake-Basins by the 

 lower ends of Glaciers, where they may have formerly ex^^anded 

 over a more level country, beyond the confines of their parent 

 valleys. It appears to be admitted that a great many lakes lie in 

 positions where it would be natural to suppose them so formed, if 

 only glaciers could have possessed the erosive powers necessary for 

 the purpose. The positions of the deeper parts of the lakes also 

 accord well with the theory. 



Mr. Bonney, than whom there are few, if any, more competent 

 glacialists, says, in the last Number of this Magazine (p. 197), that 

 his late observations have confirmed him in " regarding a glacier as an 

 agent of abrasion rather than of erosion," and that, "where the ice 

 rests on the level or stony plain," the subjacent materials, so far as 

 he could make out, " are little if at all disturbed " ; and he " feels 

 convinced great caution must be exercised in referring to their 

 action a rock-basin of even moderate dimensions." Without in any 



