DAVIS: THE HARVARD GEOGRAPHICAL MODELS. 
97 
structure, slope, and climate; an inch in from one to ten centuries 
may be taken as a rough value, averaged for large areas. Thus a 
simple introduction is given to the essential idea of the geographical 
cgcle , the long time-interval necessary for the complete wearing 
down of a land mass to baselevel. 
Where the extended rivers cut down their vallevs across the 
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plain, they disclose part of the foundation on which the sands and 
gravels lie for a little distance outside of the oldland shore. The 
foundation is seen to be nothing more than an extension of the old- 
land, whose surface pitches seaward at a somewhat steeper angle 
than the slant of the coastal plain. The strata of the plain are thus 
seen to constitute a wedge-shaped mass (shown in section on the side 
of the model), with the thin edge of the wedge at the oldland shore 
and thence thickening seaward. 
While the rivers flow upon the resistant rocks of the oldland 
foundation just outside of the former shore line, they find the work 
of deepening their valleys much more difficult than when they reach 
the weak strata of the plain, further down stream. Hence there 
will be, for a certain period in the history of the plain, a relatively 
strong descent of each river as it passes from the oldland rocks to 
those of the plain, and this strong descent will cause rapids or falls. 
A line drawn through the rapids of all the extended streams may be 
called the fall-line: it lies near and about parallel to the oldland 
shore. It is important as marking the head of navigation in the 
larger rivers, and as furnishing waterpower for manufacturing 
industries. 
Taking advantage of the permission given by the relative uplift 
of the land to cut deeper than before, the streams of the oldland are 
all entrenching themselves in their former valley floors. This 
change is as yet hardly noticeable in the small streams and in 
the headwaters of the larger ones, but it is plainly apparent in the 
lower courses of the master rivers, where distinct terraces show the 
remnants of the former Hood plains. The terraces often serve as 
excellent sites for villages and roads, out of reach of floods. 
Simply in order to introduce a greater variety of form in the second 
model, a small volcano and several lava Hows have been built up on 
the oldland near the old shore line. The crater of the chief cone 
has a little crater within it, and a subordinate cone has been formed 
on the flank of the larger one. The lava flows radiate from the 
chief cone, following the slopes of the land before them. One flow 
