St. Francois river flowing between in a narrow gorge, known as the shut-in. They rise out of the limestones plain and have little or no sandy material on their slopes. The northern end of the section touches the central granite field.

Tin Mountain Section-This section extends in and east and west direction from Matthews creek to Smith mountain, connecting at the latter point with the section running to mount LeSueur. The most extensive marble beds within the area of the sheet are immediately west of Tin mountain, and the relations with basal sandstone are well shown. A large basic dike or boss occurs near the base of the mountain on the south bank of the Little St. Francois river. A similar dike also exists a couple of miles to the eastward.

Doe Run Section-The mines of this district are in the extreme north western corner of the sheet. The cross section extends north and south near the western boundary of the sheet, from Doe Run to the Wachita and Big Hill section. It crosses an area in which the limestones rest directly upon the granite. Considerable sandy material together with crystalline bowlders occupy the depressions in the granite, and this is also the ore horizon. Further details are shown in a section taken in a little different direction (figure 19) and also a smaller cut representing the formations existing at the shaft (figure 20).

DEFORMATION.

Although the area forms a part of the Ozark uplift, where dynamic forces have operated on a large scale, the bowing of the strata has been regional in character and the local effects have not been marked. Of folding there has been practically none that is perceptible within the district. Only unimportant undulations have been observed. As a rule, they are so slight that they cannot be detected without careful measurements being made. The marked inclination of strata which has often been ascribed to low folds, is the result of peculiarities in the original deposition of the beds, and not of deformation after sedimentation had ceased. Very noticeable dips are apparent where the stratified rocks rest upon the ancient crystalline bills.

UNCONFORMITIES.

In no part of the state is discordance between formations more marked than in the Mine la Motte sheet. The most marked irregularity in the even succession of the rocks is between the crystallines and the sedimentaries. It is clearly shown in all the cross-sections and in numerous other sections of which mention has been made of only a few.

Cross-section through Doe Run Mines.

FIGURE 19.

The physical break between the formations of the two classes of rocks is very much more than an ordinary unconformity in which, owing to a warping of the earth's crust, the parallel. ism between two formations is not preserved. The surface upon which the sediments were laid down was very uneven. (See plate vi and also plate ix.) It was manifestly an old land surface deeply cut into valleys and ridges, presenting even greater contrasts of relief than is indicated in the present topography. The juncture of the sedimentary formations and the crystallines is nearly every where marked by a bowlder bed, or conglomerate, of greater or less thickness. Usually there are only a few feet of conglomerate, though bowlders are often found as much as 50 or 60 feet from the base. The strata resting on the conglomerate may be sandstone or limestone-usually the former. The characteristic arrangement is shown at the Doe Run mines (figure 19). At the shaft which was sunk on top of a steep granite knoll, as shown by drill-holes, the limestone rests directly upon the crystallines. The details are shown in the accom. panying cut (figure 20).