560 CENOZOIC TIME. 



rivers (the original condition of the terrace-plains) often differ much 

 in height in different parts ; (2) the rains and streamlets often wear 

 away the soft material of the terraces, diminishing their height, and 

 sometimes obliterating the plain altogether, or reducing it to a region 

 of hills, or " horse-backs ; " (3) the winds carry off the light soil of 

 the surface, and, in the course of centuries, may produce great results. 



Again, the terraces of small tributaries, at a distance from the river 

 into which they flow, are lower than those of the latter, because both 

 their floods and their eroding power are less. 



Again, when there are rocks in the course of a stream, a terrace 

 above the rocky barrier may differ in height from its counterpart 

 below ; because the stream is unable to wear down its bed, and is 

 more or less dammed up by the barrier. 



It should be observed that the deposits terraced in North America are those of the 

 Champlain period, formed as explained on pages 545, 547 ; and consequently that the 

 beds of the uppermost terrace, instead of being the oldest, are usually newer than, 

 those of the lower, because they are the upper part of the Champlain formation. The 

 formation in any place is often wholly Diluvian, and sometimes wholly Alluvian; and, 

 in either case, the upper beds are the newer. Where the Alluvian deposit along 

 the middle of a valley has been laid down over the earlier Diluvian, in the manner 

 remarked upon on page 547, and then removed again down to the level of a lower 

 terrace, the deposits of the upper terrace would be older than the rest. Again, if, as in 

 Europe, a second epoch of ice and change of level lias following the Champlain period, 

 there may be elevated alluvial formations along the sides of a stream, which are of 

 later origin than those of the Alluvian era. Alluvial beds of Modern time are seldom 

 present, unless locally, on any of the terraces excepting the lowest, or that constituting 

 the existing river-flats. 



Terraces along rivers or lakes are uncertain evidence as to the amount of the elevation 

 which occasioned their formation. Only the terraces of large open valleys give even ap- 

 proximately the truth; and not these, unless the bed of the channel is alluvial quite to 

 its mouth, so that it is nearly certain that the river has, since the elevation, excavated 

 its bed to the same slope it had before it. If there are falls in the stream, or descents in 1 

 rapids over rocks, amounting, say in all, to seventy-five feet, the excavation would in 

 most cases be so much less than the amount of elevation; and this should be added, in 

 order to get an approximate conclusion. There is also an unknown amount to be sub- 

 tracted, on account of the greater slope of the flood-waters of the Diluvian era than that 

 of modem floods; for the slope of a flooded stream does not depend on the slope of its 

 bed simply, but both on that and on the amount of water supplied in the flood ; and, as 

 already stated, the slope, in such a case, varies greatly with the character of the valley: 

 a narrow and obstructed gorge in the course of the valley causing a great rise of the 

 flood-level above it. 



Other considerations bearing on this subject, and essential to right conclusions, are 

 presented in the chapter beyond, on Rivers. 



2. Geography The elevation which occurred at the opening of 



the Recent period made a vast change in American Geography. The 

 New England and Labrador coasts gained much in extent ; and Nova 

 Scotia was again united to the main land. The one immense interior 

 lake became the five Great Lakes ; and hundreds of smaller lakes,., 

 along the rivers and elsewhere, disappeared. The Kankakee Swamp 



