Do.lpe.] 258 (M:irch2i, 
the river bod wliich it faces. On tlio side toward tlie stream 
the plain is bounded l»y an escarpment, the two together mak- 
ing tlie terrace : tlie opposite side of tlie ])lain is usually bounded 
by more elevated land, either an older and higher terrace; oi- the 
true valley wall. Terrace plains represent approximntely the 
level of the stream at the time of their formation, while the 
height of the escarpment shows the depth to which the river cut 
before forming another plain. 
Classification of Terraces. 
The topographic form sketched above includes all river 
terraces, divisible, however, according to their origin into two 
great classes ; terraces of planation, formed in the solid terrane 
of the region ; and those formed in the loose alluvial dejiosits of 
the river valley, which are termed alluvial terraces. I shall 
devote my attention to the latter class only. 
Alluvial Terraces and Alluvial Plains. 
Alluvial terraces are formed by streams cutting down into the 
loose waste accumulated in their valleys, usually in the form of 
alluvial plains. Thus alluvial terraces and alluvial plains are 
very intimately associated, and a consideration of the former 
necessarily includes a consideration of the latter. An alluvial 
plain represents one of the forms assumed by the M'aste of the 
land on its w:ay to the sea ; an alluvial terrace is then a sec- 
ondary form imposed on the waste of the land under certain 
conditions which I shall consider later. Among other character- 
istic forms in addition to alluvial plains may be mentioned allu- 
vial fans, talus slopes, etc. Any one of these considered in its 
relation to the river would furnish interesting matter for a 
separate paper. 
An alluvial terrace is formed in the loose alluvial materials of 
a river valley as the result of three actions which occur in the 
following order ; first, an initial valley is cut in a land mass ; 
secondly, that valley is at least partially filled with waste ; and 
