94 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
of the monoclinal ridges, in contrast to tlieir graded back slopes; 
the retreating shore line where the sea is actively cutting cliffs, in 
contrast to the advancing shore line where the main river is building 
forward its delta. A slight and slow depression of the region 
may be inferred from the breadth of the flood plain in the main 
valley. All these matters of detail may serve as the basis of 
observation, description, and explanation. When accompanied by 
illustrations, through maps or photographs showing actual examples 
of similar features, the model may be made the basis of effective 
teaching. 
On all mountainous coasts, where the streams are actively at 
work washing the Avaste of the land into the sea, and the waves 
are vigorously cutting back the headlands, the sea floor must receive 
abundant supply of sediments. Judging by the depth of the valleys 
and the height of the shore cliffs in the first model, these processes 
must have been going on for ages. The sea floor near the shore 
must be strewn over with heavy sheets oh land waste. With this 
introduction, the second model may be presented to a class of 
young geographers. 
Second model . 1 Narrow coastal plain between mountains and 
sea. This model exhibits a region where the piedmont hills descend 
abruptly to a smooth lowland, and the lowland or coastal plain 
slopes very gently to the sea. Soundings show that the inclination 
of the lowland"is continued in the slowly deepening sea floor. The 
conditions of life may here be much more favorable than in the 
«/ 
previous example, provided the climate is fitting and the soil fertile. 
A large population may gather on the lowland, enjoying the contrast 
between the mountains in the background and the sea in front. 
The plain is divided into many similar parts by the shallow valleys 
of streams that flow across it from the mountains to the sea. Each 
division of the plain may be called a doab ; that is, a two-river space 
( doctb , a term of Hindu origin, meaning “ two rivers ”). The 
surface of a doab is so smooth and nearly level that a great part of 
the rainfall enters the pervious soil instead of running off in surface 
streams, as is the habit in the steep ravines and valleys among the 
1 In order to gain room for tlie coastal plain in this model, the higher mountains in the 
back country of the first model are cut off and the sea front is extended. The modeled 
area is, as it were, brought forward. 
