102 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
more acutely pointed. From this more irregular outline, the shore 
is really becoming smoother by the action of existing processes. 
It is moreover manifest that the land cannot have stood long, as 
the earth counts time, in its present position ; for if so, the headlands 
would have been more cut back by the waves, and the bays would 
have been more filled by delta growth. A great deal more time 
must have been spent in wearing the valleys between the ridges 
than has been spent in building the little deltas. Thus it becomes 
clear that the land has had a much longer history than is recorded 
by the cliff cutting and the delta building; and that by some other 
processes than those of today was the original irregular shore line 
produced. 
A simple and sufficient explanation of both these conclusions may 
be found by assuming that the valleys were eroded while the region 
as a whole stood higher than today, like the first model; and that 
then a relatively uniform movement of depression occurred, whereby 
the sea advanced upon the lower lands. It necessarily entered 
furthest into the main valleys, and by drowning them produced the 
branching bays between the advancing headlands. This explanation 
fits the case so perfectly that there can be no doubt about it. Since 
the movement of depression occurred, a relatively brief period of 
time has elapsed; for, as has been already said, the deltas are small 
and the sea cliffs are low. When more time has elapsed, the head¬ 
lands may be cut far back and the deltas built far forward; and 
then the shore line will be much simpler than now. 
The lessons taught by comparing the second and third models 
with the first are among the most important that the young 
geographer can learn. In the first place they give him a rational 
introduction to the fact that the lands rise and fall with respect to 
the sea, and that in consequence of these changes, the face of the 
earth alters its expression. The subject may become scientific not 
only in its content, but in its treatment. Indeed, long before the 
deliberate advance of the description and explanation here suggested, 
any intelligent school boy or girl, looking at the first and third 
models, would have seen that the latter was derived from the 
former by partial submergence followed* by slight changes of the 
shore line. It is remarkable that an explanation so simple and 
sufficient as this should have been so long making its way into the 
school room. Yet it is only in recent years that teachers have 
taught that bays like the Chesapeake, or firths like that of the Clyde, 
