1S94-] 263 rOodge. 
development of rivers. It might be objected, in view of wliat is 
said above, that such a terrace could have no value as the type of 
normal terraces ; but this criticism does not really weaken the 
classification as nothing is really built up from it, and also it is 
just possible that such a terrace may be formed under certain con- 
ditions ; and it has a decided value for comparative purj)0ses. 
Another way in which a normal alluvial terrace might be formed 
is as follows : suppose tliaL a stream wIkjsc load is slightly 
in excess of the carrying power ac(juire by capture the 
headwater portions of another stream. In the district thus 
acquired, as in the headwaters of all streams, there would be a 
greater excess of carrying power over load than in any other 
part of the stream, consequently the capturing stream would 
probably have its carrying power greatly increased without any 
corresponding increase in load, and the stream would [)robably 
begin to cut into any previously de[)Osited alluvial plain and 
terrace it. 
A third process by which a normal alluvial terrace might be 
produced would be where a stream which had been working 
mostly in soft rocks cut down into harder rocks underneath the 
softer ones. The effect of the difference of hardness in the two 
beds, or sets of beds, would be a decrease in the load furnished 
to the stream in a given time, and conse(piently a decrease in the 
amount of work which tiie stream would liave to do in order to 
maintain an unobstructed course. Hence, without any increase 
in volume the stream might be able to terrace an alluvial [)lain 
formed Avhile attacking soft rocks. 
We will now pass from terrace-making processes that necessarily 
occur in any river's history to certain ones which would not 
necessarily occur in the develo|)mentof a river, but which, if they 
<lid occur, would not be abnormal. Terraces developed by such 
action may be called subnormal terraces. 
Subnormal Alluvial Teuraoes. 
A subnormal terrace might be <lue to certain climatic changes 
in the basin drained by the river. Suppose for instance that the 
amount of annual precipitation in a region should begin to 
decrease, and we know it has decreased in recent times in certain 
