116 Drift formed on Land. 



sequently stratified deposits is so great, that they rest every- 

 where imconformably upon each other, shewing distinctly 

 the diflPerenoe of the agency under which they were accumu- 

 lated. This unconformable superposition of marine drift 

 upon glacial drift is so beautifully shewn at the above-men- 

 tioned locality near Cambridge (see diagram, p. 114.) In this 

 case the action of tides in the accumulation of the stratified 

 materials is plainly seen. 



The various heights at which these stratified deposits oc- 

 cur, above the level of the sea, shew plainly, that since their 

 accumulation the main land has been lifted above the ocean 

 at different rates in different parts of the country ; and it 

 would be a most important investigation to have their abso- 

 lute level, in order more fully to ascertain the last changes 

 which our continents have undergone. 



From the above mentioned facts, it must be at once obvi- 

 ous that the various kinds of loose materials all over the 

 northern hemisphere, have been accumulated, not only under 

 different circumstances, but during long-continued subsequent 

 distinct periods, and that great changes have taken place 

 since their deposition, before the present state of things was 

 fully established.* 



To the first period, — the ice period, as I have called it, — 

 belong all the phenomena connected with the transportation 

 of erratic boulders, the polishing, scratching, and furrowing 

 of the rocks, and the accumulation of unstratified, scratched, 

 and loamy drift. During that period the mainland seems 

 to have been, to some extent at least, higher above the level 

 of the sea than now ; as we observe, on the shores of Great 

 Britain, Norway, and Sweden, as well as on the eastern 

 shores of North America, the polished surfaces dipping under 

 the level of the ocean, which encroaches everywhere upon 

 the erratics proper, effaces the polished surfaces, and remodels 

 the glacial drift. During these periods, large terrestrial 

 animals lived upon both continents, the fossil remains of 

 which are found in the drift of Siberia, as well as of this 

 continent. A fossil elephant, recently discovered in Vermont, 

 adds to the resemblance, already pointed out, between the 



