Page:The National geographic magazine, volume 1.djvu/40

 values be perceived that are essential to good study. The illustrations should be of actual scenes; or, if designs, they should be designed by a geographic artist. The descriptions should wherever possible be taken from original sources, in which the narrator tells what he saw himself. It is, to be sure, not always possible to know what kind of a form he describes, owing to lack of technical terms, but many useful examples can be found that may then be referred to their proper place in the system of geographic classification that is adopted.

I shall consider only one example in detail to show how far short, as it seems to me, geography fails of its great opportunity, both as taught in schools and as applied in after life.

In northeastern Pennsylvania there are several water-falls that leap over tilted beds of rock. Such falls are known to be of rare occurrence, and we may therefore inquire into the cause of their rarity and the significance of their occurrence in the region referred to.

We may first look at the general conditions of the occurrence of water-falls. They indicate points of sharply contrasted hardness in the rocks of the stream channel, and they show that the part of the channel above the fall has not yet been cut down to base level. When the channel reaches base level there can be no falls. Now it is known from the general history of rivers that only a short part of their long lives is spent in cutting their channels down to base level, except in the case of headwater streams, which retain youthful characteristics even through the maturity of their main river. Consequently, it is not likely that at any one time, as now, in the long lives of our many rivers, we should see many of them in their short-lived youthful phase. Falls are exceptional and denote immaturity. They endure a little longer on horizontal beds, which must be cut back perhaps many miles up stream before the fall disappears, than on tilted beds, which must be cut down a few thousand feet at most to reduce them to base level. Falls on tilted beds are therefore of briefer duration than on horizontal beds, and are at any time proportionately rarer. On the headwater branches of a river where youthful features such as steep slope and sudden fall remain after the main river has a well-matured channel, we sometimes find many water-falls, as in the still young branches of the old Ohio. These are like young twigs on an old tree. But even here the rocks are horizontal, and not tilted as in the cases under consideration.