1006 The American Naturalist. [November, 
measuring recent terrestrial elevations. Since that meeting I have 
gone over many critical localities and the phenomena confirm the 
conclusions then announced. The importance of this contribution is 
not so much in a determination of the magnitude of post-glacial elevation 
as in finding a means of physical measurement of it and in my consequent 
challenge of the doctrine of ice dams in the late formation of high-level 
beaches and terraces. For no apparent reason has the structure of the 
terraces escaped early observation to such a degree that hitherto it has 
not been described in such a way as to be used as a meter of recent 
terrestrial changes of level. 
The structure may be briefly set forth, The terraces are not those 
of the sloping rivers, but are the much more horizontal remains of 
water plains. The platforms do not merge from one step to the next 
below and thus make the ancient slopes of the rivers as has been 
often assumed, but they abruptly descend as steps to the lower plains. 
Thus a small meadow widens out into a broad flat, with the river near 
the surface of the plain along the upper part of the flat, but further 
down, it descends to greater depths below the same floor or plain, which 
on being eroded become a lateral terrace hounding the still lower plains. 
Thus as meadows, plains and remanie terraces, the same platforms may 
often be traced for many milesin length, disappearing owing to erosion, 
and to the distance of the terraces from the source of supply of sands 
and gravels. The terraces often cross the country and extend from one 
valley to another. Subject to certain corrections, these meadows, flats, 
and terraces mark the lowering of the base planes of erosion, or in 
other words indicate the elevation of the land. That is to say, the land 
has approximately been elevated as much as the sum of the heights of 
the terrace-plains one above the other. In some places, these are 
situated only a few feet apart in elevation, yet in other localities several 
of the steps are so combined that the great terraces may be from 50 to 
250 feet above the river. Occasionally, in the course of a few miles, 
scores of terraces, may be ascended or descended and counted with 
certainty. Yet at any one locality, there are seldom more than four 
or five lateral terraces distinguishable; but these four or five are not 
identical with the four or five platforms observed several miles away, 
in the same great valleys. 
Such distinct terraces are seen to an elevation of 2700 feet at the base 
of Mount Washington, with terrace material much higher, but without 
the preservation of the structure upon the steep mountain slopes. 
The terrace forms described have now been observed under so many 
