western (plate iv). The northern district forms a part of the drainage divide which separates the waters which flow northeastward directly into the Mississippi river from those which flow southward into the St. Francois.

The foundations of the upland are the hardest rocks; in the southwest the porphyries and in the other parts cherty limestones. This region is deeply cut up by the streams, the principal one being the St. Francois river, which traverses the area in a general north to south direction. The tributaries are small, numerous, torrential. The valleys are contracted, steep, and the floodplains very narrow. The porphyry hills rise in high, irregularly rounded mounds, with precipitous slopes. When the watercourses wash their bases or cut across them narrow gorges are formed often with perpendicular walls.

The persistent strength of the other portion of the upland surface, in the northeast, is a hard, very cherty limestone which overlies softer, less silicious beds of limerock. Beyond the limits of the district the upland is continuous, and the portions appearing within the boundaries are long lobes or extensions which project out from main body. These ridge-like elevations terminate abruptly in steep slopes which form sections of a rather pronounced escarpment, of tortuous course, but which has a general trend of northwest and southeast. The sides of the declivity present sharply incised or crenulated outlines. The hills formed are thickly covered by the hard cherty fragments, which obscure or totally hide the strata underlying. Several prominent chert-covered hills which occupy isolated positions beyond the boundaries of the escarpment appear to be remnants of the steep slope at a time when it extended out into the lowland much further than at present. These solitary mounds often present a striking contrast to the surrounding country, and form conspicuous features of the local landscape.

Farmington Lowland. The median plain which occupies a large portion of the district is a part of a broad zone that extends over a much more extensive area. Within the limits of the sheet this belt runs in a direction nearly northwest and southeast. It includes most of the northeastern two-thirds of

the sheet. It is limited on the east by the irregular cherty escarpment of the upland forming the divide between the Mississippi and St. Francois rivers. On the west the porphyry hills interrupt its continuity.

The general elevation of the plain is about 1000 feet above mean tide. This level is 800 to 900 feet below the level of the great peneplain.

The rocks of the Farmington plain are soft, or at least much more easily succumb to meteoric influences than do the por phyries and the cherts. Its principal features are the outcome of rather vigorous erosive action upon limestones and sandstones, and its general surface is rolling. Wherever the calcareous beds predominate the topographic outlines are rounded and greatly softened; where sandstones occur the relief is bolder. In the lowland the river valleys have a tendency towards a broad open type, in contradistinction to those of the upland where the narrow contracted gorges prevail. The extent of the plain, however, is not confined to the areas of limestone and sandstone, but with some modifications covers also the granites. The latter disintegrating much more readily than the porphyries leave the areas occupied by the fine grained rocks standing far above them. The surface relief of the granite district is noticeable more rugged than that of the sedimentary areas, but the comparative resistance of the two kinds of rock are not so diverse but that the areas occupied may be grouped together.

The lowland is a plain of denudation. It is the product of a former cycle whose work was interrupted before completion. In point of time this cycle was a later one than that represented by the Tertiary peneplain, and immediately preceded the present one. The effects of the present cycle of erosion are well shown in the sharply cut trenches in the plain where erosion has recently been accelerated with vigor.

Special Features.-The Mine la Motte district exhibits with exceptional clearness certain physiographic features of great interest. These are superposed waterways; and the particular phenomena they present are so conspicuous that they have at

tracted the attention of the inhabitants, who have applied to them the name of "shut-in." At the point of the "shut-in " the stream-valley which is usually quite wide and shallow, with flood-plains on both sides perhaps, abruptly contracts to a narrow gorge with steep sides. The watercourse continues to flow in the deep cut for some distance, when the valley rapidly widens out again to its former dimensions, and assumes the same type of relief that it had above the "shut-in."

An examination of the lithological characters of the banks of the river shows that both above and below the point of the "shut-in" the rocks are very much softer and much less resistant than at the contracted place, being sandstone or limestone, while at the gorge the hard porphyry appears. The "shut-in" is thus a point where a stream which has been flowing over relatively soft rocks crosses a belt of very hard material. One of the best examples of a "shut-in" shown in the Mine la Motte area is on the Little St. Francois river about two miles west of Fredericktown. The stream as it enters from the limestone district into the porphyry passes between two high hills, the channel being at the bottom of a very sharp V-shaped gorge, the sides of which are the steep-sloped mountains Devon and Buckner. The watercourse continues for a distance of more than a mile through the canyon before it again emerges into its broad valley composed of limestone.

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In the annexed sketch the features of the shut-in" are admirably brought out. The rugged topography of the porphyry area with the river bisecting it is strongly contrasted with the flatter relief of the less resistant limestone areas. Even the subordinate tributaries of Slater and Mill creeks have wide valleys compared with the "shut-in." (Figure 4).

Five miles farther to the west the same stream enters another porphyry area. A "shut in" is formed in which the river runs for a distance of more than two miles to its mouth, where it opens into a similar gorge of the St. Francois. Pine mountain rises on one side, and Tin and Hawkin peaks on the other.

The "shut-in" on the St. Francois river at the mouth of the Little St. Francois is about one and one-half miles long, and runs at right angles to the other one just mentioned. Upstream are the less resistent granites, and downstream the softer sandstones and limestones.

Similar physiographic features exist just outside the limits of the sheet in the Iron mountain district. The "shut-in" on Stout creek, two miles east of Arcadia, is one of the most notable.

The explanation of the "shut-in" is found in the rivers being superposed streams, whose courses were inherited from a former cycle. As the work of erosion was begun after the uprising of the Tertiary peneplain, the streams corraded their channels rapidly along the courses which they first assumed. In the wearing away of the rocks, of the limestones and sandstones, the old porphyry knobs were finally uncovered, and the streams encountering these cut directly across them. In order to maintain their grades they eroded deep, narrow channels in the same period that in the softer rocks wide valleys were formed.

DRAINAGE.

The general drainage of the Mine la Motte region is southerly in direction. Two streams, the St. Francois and the Little St. Francois rivers, are the principal waterways. They unite near the southwestern corner of the sheet. Only a very small

area, in the extreme northeastern corner, does not belong to the principal or St. Francois drainage system.

The St. Francois river rises a short distance north of the limits of the sheet, and flows south in an irregular, tortuous course through the western third of the area. For the most part the valley is narrow with steep sides and no flood plains. In the upper portion of its course it is broader, more open, with flood-plains of considerable importance. This part of the channel is in the less resistant limestones, and is nearly down to grade. The decline is not more than four or five feet to the mile. Elsewhere the stream is acting vigorously upon the rocks, cutting them down rapidly. The slope of the bed is 20 to 30 feet to the mile. In the more contracted places the descent is very much more, producing rapids and falls. The principal tributaries are Doe Run, Wachita, Wolf, Turkey and Pine creeks. All are small streams; the first being about eight miles in length, and the last three not more than four or five miles long.

The Little St. Francois drains over one-half of the area. It rises in the northeastern part of the district near Libertyville, flows southward to Fredericktown and then abruptly bends, running nearly due west to its juncture with the larger stream in the southwest. Most of its course is in the less resistant limestones and the valley is consequently open. Only at its mouth and near Fredericktown is it contracted to a narrow gorge. Directly east of Knob Kick the river traverses the eastern edge of a saudstone belt for several miles; and the west banks are very steep, but on the opposite side where the limestones prevail the slope is gradual. The chief creeks flowing into the Little St. Francois are Wills, Musco and Fritzzell on the west, and Rock, Village, Mill, Slater, Matthew and Cedar on the east and south.

STRATIGRAPHY.

GENERAL GEOLOGY OF THE REGION.

The district covered by the Mine la Motte sheet contains the oldest rocks of not only Missouri, but of the whole Mis