such infantile characters long. The steep walls are gradually worn back, and the few original consequent tributary streams, having cut down their lower channels to the grade of the main stream, begin to push back their headwaters. New side streams spring up along the walls and slopes of the gorge, cutting deep scars and seams in them, and thus hasten their recession. As the number of tributaries increases the broad flat divides are narrowed and even begin to lose their flatness. The lakes which formed at first have their outlets cut down and are drained, while the channels of the older streams, which were rough and broken by falls and rapids, gradually lose their inequalities. The volume of the main stream is somewhat increased by the growing number of side branches, but as each one of these comes down laden with the debris which its active little headwaters and its steep banks furnish, a great load is soon added to it. All of this load the larger stream cannot manage to transport, and so some portion is dropped at the mouths of the several tributaries, forming cone-like alluvial deposits that project into the main valley, while part is taken by the master stream and is used by it to steepen its slope, thus enabling it to carry off a greater load. Many streams in this stage may be found among the high lands of the Sierras, the Himalayas and other regions of plentiful rainfall and recent elevation. Excellent examples may be found in certain portions of Southern Maryland. In St. Mary's county numbers of the southwestward flowing streams show these adolescent features, with the over-loading of the main stream and consequent flood-plain building.

The constant increase of the catchment area by reason of the ever growing number of streams and the pushing back of the headwaters, continues until the divides between opposing streams, whether of the same or of different drainage systems, are sharp and steep. The ramifying branches have sought out every square mile of territory, so that the whole region is completely drained. The small headstreams thus having no new territory to conquer by linear development begin to reduce the steepness of their own slopes, to soften their valley sides, and to reduce and round off a little the tops of the hills. Thus

the amount of mechanical sediment brought to the larger streams decreases while the volume of water still remains about the same. The main channels smoothed out still more are so far reduced that they describe smooth regular curves from source to mouth. Up to this time the slope of the river channels has been slowly changing, but it now reaches a period of comparative stability, since the changes in load and in volume, which are the factors determining the curve, are very much slower hereafter. The channel slopes are now more permanently suited to the needs of the streams and the latter may be said to have established graded channels. The accompanying figure shows the slope to be steepest at the source, but to rapidly decrease to a midway point whence it is of constantly but very gently decreasing fall to the mouth. As all the streams gradually approach such a graded condition, the inter-stream areas forming the divides also gradually wear down. Such an area is included within the

FIG. 3.-A normal stream profile (after Penck).

Johm.

boundaries of the Piedmont Plateau on Plate III. Similar topography characterizes most of Northern Virginia and large portions of Eastern New England. The country and its drainage may be said to have reached its Maturity.

The gradual change in topography and in drainage which have just been briefly sketched presupposes, first of all, that the land and sea have remained constant to each other long enough to permit such development to occur. This supposition is not always justifiable, since multitudes of cases can be cited to show that after a period of rest long enough to permit of the topography developing to some stage earlier even than Maturity, earth movements have closed what may be called the current cycle and have inaugurated a new one. In order to make the series of topographic forms complete, however, some students have carried the scheme beyond the stage of Maturity and described yet another and final stage which has been likened to Old Age.

Suppose that after Maturity is reached, the same conditions endure for an indefinite period. The streams would still continue to deepen their channels although at an ever decreasing rate. The hills and mountains would gradually sink lower and yet lower, yielding now more to the solvent action of the waters charged as they would be with acids from the mantle of soil and vegetation which covers everything. Finally all slopes would be reduced to the lowest possible angles and the divides also would be very insignificant, except at points far from the mouths of the streams. The lowest zone would be along the sea-coast, where the land would be reduced quite to sealevel. From here inland there would be the slightest possible rise in order to permit the rains to really drain away and not gather into stagnant pools. The whole district would be nearly featureless and so closely approach a plain in appearance and contour that it might appropriately be called an almost-plain or peneplain, just as an almost-island is a peninsula.

It is obvious that the lowest level to which a land can be worn down by stream action is sea-level, and even this can never be reached except at the very shores, since some slope is needed to carry off the water. Therefore the ocean is called the great baselevel, or the base down to whose level all the forces of Erosion or Denudation are working to reduce the land. Local base-levels may exist for a time, such as the level of a lake, which is the base-level for streams entering it, or the level of a stream where it crosses an unusually resistant stratum, which may be the base-level for its tributaries above this point. But eventually all the streams are controlled by the level of the sea. Such an enormous duration of time, throughout which the position of the land would have to remain fixed with reference to the sea-level, would be required, however, to permit of the production of such a complete peneplain, that there is scarcely any warrant for supposing that such a condition has often existed. Nowhere to-day can an example of such a topographic feature be found.

On the contrary, everywhere there is evidence to show that the land and sea do not long continue constant to each other. Young as

are the Coast Ranges of California, they had, since their elevation, attained very nearly to ripe Maturity, when great subsidences took place, drowning part of them. These accidents again were recently followed by successive lesser re-elevations. The eastern coast of North America has suffered repeated elevations and subsidences since the period of the last great elevations of the Sierra Nevada, and is still undergoing slight oscillatory movements. Other instances might be cited to show that the chances are probably small for a locality to reach even to the perfection of well-matured topography.'

Although the topographic cycle has perhaps never had an opportunity to run its full course, yet it is convenient for the purposes of understanding and explaining topographic forms to retain the conception of a complete cycle, which might be renewed, if, after attaining to the stage of a peneplain, the land were again elevated and the streams commenced their tasks anew. As we have seen, however, the rule is that at some stage in the ideal cycle the march of development will be interrupted. Such interruption may result from one or several causes. The most common interruptions come from re-elevation of the land, whereby the streams receive increased. energy, or from depression, which allows the sea to invade a portion of what was dry land and reduce the energy of the streams by decreasing the height from which they have to fall to reach sea-level. When by reason of the rise of the land the streams renew the vigor of their own cutting, and begin to cut canyons below the general surface which they have before produced, they are said to be revived. The same phenomenon would be produced if, after long delay, the master stream of some system should succeed in cutting through a stubborn ledge and begin to work rapidly down through a more yielding understratum. It will appear farther on that most of Maryland's streams show the reviving effects of re-elevation. The illus tration forming Fig. 2 shows a district of revived drainage. Depression whereby the lower courses of most of the rivers are submerged beneath the sea hastens the reduction of what is left above. sea-level, and by decreasing the slope of the lower courses often

1 See R. S. Tarr, "The Peneplain." Amer. Geol., vol. xxi, 1898. pp. 351, et seq.

causes the building of flood-plains at these points. The coasts of Maine, of Norway, and of Maryland afford excellent examples of such topography, which is called drowned.

Migration of Divides.

The progressive development of a piece of country through the stages of a Topographic Cycle is accompanied by many interesting processes, some of which will be considered in this and the following

section.

When the broad flat divide which characterizes the infancy of stream growth is converted to the sharp serrated crests and ridges of earliest maturity, the streams, which before were battling against a common enemy, viz. the unreduced land mass lying between them, are then brought into closer rivalry. Each stream heading against a divide is endeavoring to wear it away and to gain more drainage area. If the streams are pretty evenly matched, then the divide must gradually sink down, until it becomes a low ridge almost exactly beneath the line along which the headwaters of the opposing streams first met on the surface of the plateau. Should it happen, however, that the streams on one side of the crest had an advantage over the opposing set then the rocks would be worn away unevenly on the two slopes; the stronger streams would wear away their side faster and the divide would move towards the weaker set of streams.

There are many ways in which one set of streams may come to have more power than an opposing set. The favored streams may have a shorter course to the sea, thus giving them a steeper slope, or what may amount to the same thing, the course may lie on softer rocks which, being more nearly reduced to the sea-level or base-level along the lower course, concentrate the greatest possible amount of steep slope at the headwaters. This is excellently illustrated in Maryland by the contrast in slope which exists between the tributaries of the Monocacy, a stream situated on easily eroded slates, sandstones and limestones, and the main streams of the Patapsco, the Patuxent and other rivers which have to cross the resistant gneisses and other crystalline rocks of central Maryland. Again, greater rainfall will give to one side larger volume and greater cutting powers. Excel