The faunal features of the limestones are not conspicuous. With the exception of one or two places, fossil remains are very meager. The Linguella beds, best exposed at the Jack diggings on the Mine la Motte estate, lie about a dozen feet below the principal dolomitic number, and just above the upper ore horizon. In the thin blue shales the layers are often crowded with shells. Only a single species, Linguella lamborni ( Meek), has been found. In the gray limestone at the Doe Run mines a similar but smaller form of Linguella has been obtained in the cores of the diamond drill, at a depth of over one hundred feet. Near Fredericktown fragments of fossils have also been obtained. The peculiarities of the Linguella shells has assigned the beds containing them to the Upper Cambrian.

LESUEUR DOLOMITE.

Above the magnesian beds of the Fredericktown formation there is a very cherty dolomite which, within the district, attains a thickness of fully 250 feet. The name is derived from a conspicuous hill made up almost entirely of the formation, and lying on the headwaters of Rock creek. It is called after the discoverer of lead in Missouri. The chert occurs in nodules and nodular layers and in places makes up over one-half of the bulk of the formation. In breaking down under the action of atmospheric agencies the chert-bearing beds resist weathering influences much longer than the non-silicious underlying limestone. As a result the high grounds which are composed of the beds of this formation are deeply covered by chert, so that the local appellation of "flint-hills" is a very appropriate term. The underlying members the non-cherty Fredericktown limestone wears away much faster than the LeSueur beds, and as a consequence wherever the former beds are overlain by the latter, a more or less abrupt escarpment is found which constitutes one of the most marked physiographic features of the region. In many places over the area occupied by the Fredericktown limestone and from which the LeSueur formation has been removed through erosion there are often thick deposits of

residuary chert. It doubtless represents all that remains of the beds belonging to the higher horizon.

In lithological character the LeSueur beds are sharply contrasted with the underlying strata. The layers are not so thick, the presence of large quantities of chert giving the beds quite a thin-bedded aspect.

Within the limits of the Mine la Motte sheet the LeSueur limestone is not a very important formation but elsewhere its geographic distribution is quite extensive. Only a few of the rugged chert-covered hills in the northern portion of the area are composed in part of it. In the eastern part of the Mine la Motte estate the steep-sided elevations made up almost entirely of this formation rise from the channel of Rock creek. From this point nearly to the northern border of the sheet it forms an irregular belt, from one to three miles wide, within the district, but extends much farther to the east. In the northeast corner of the area there is another small development.

GEOLOGICAL STRUCTURE.

GENERAL RELATIONS OF THE FORMATIONS.

In its main features the geological structure of the region covered by the Mine la Motte sheet is simple. But while there is relatively great simplicity as regards deformation there are more or less complicated characters which are the result of discordant sedimentation. Between the two principal formations, the massive crystallines and the bedded rocks, there is a remarkable and profound unconformity which gives rise to great intricacy in the stratigraphy of the region. The stratified rocks, however, form a regular conformable sequence from bottom to top, and although the lithological characters change rapidly from place to place and from one level to another, little difficulty is encountered in making out the stratigraphical details.

For the most part the inclination of the strata is slight, usually hardly perceptible. In the vicinity of the old granite and porphyry elevations with their steep sides the slant of the

principal planes of stratification corresponds to the slope of the old buried hills, but a short distance away the beds become practically horizontal. This declination around the porphyry prominences is often as much as 20 to 25 degrees.

Notwithstanding the fact that the strata have not undergone marked folding or other profound deformation, there are considerations to be emphasized, which are of the greatest importance from an economic standpoint and at the same time of much interest scientifically.

With considerable detail there has been already given a description of the local and general lithological characters of the various rock masses. In a broad way all of these diverse masses may be brought together so as to form a few distinct groups or geological formations. Their number and general relations may be inferred from the table of formations and the general vertical sections. But the exact relations which the formations bear to one another over wide areas is displayed best by geological cross-sections along critical lines or those lines which are best calculated to disclose the many different features of both original and acquired structure.

GEOLOGICAL CROSS-SECTIONS.

Smith-LeSueur Mountain Section-Smith mountain is in the southwest corner of the district. Mount LeSueur is near the central point of the east boundary of the area. The section connecting the two elevations is about eighteen miles in length. It traverses the three physiographic belts and crosses the median one, the Farmington lowland plain, nearly at right angles to its main trend. The line of the section passes over every geological formation represented within the limits of the sheet. (See plate iii, figure N.O.P.)

The foundation of all is manifestly the coarse-grained red granite. Upon its upper uneven surface the sediments rest, most of the deposits occupying valleys or basin-dike depres sions. The stratified rocks are everywhere nearly horizontal except in the immediate neighborhood of the old crystalline elevations, where the strata rise more or less abruptly, and the

dips are often quite marked. Around the old elevations the strata slant away in all directions. The most instructive part of the section is the northern half, the portion in which the sedimentary strata are the surface rocks. The La Motte sandstone covers a considerable part of the distance between the granite area and the extremity of the section. At the point where the Fredericktown limestone begins to appear, drill records show that the sandrock attains a thickness of over 250 feet. Near Rock creek the change from sandstone is very abrupt and on the same level. Diamond drill-holes and the section along the stream at this place indicate clearly that a fault of not less than 300 feet exists, bringing the base of the cherty limestones nearly down to the level of the top of the sandstone, and almost hiding from view the Fredericktown dolomite. The details of the fault are shown in another place (figure 25).

Wachita Mountain-Big Hill Section-The course of the section is across the sheet in an east to west direction. The location is near the southern boundary line of St. Francois county. It is a section across the lowland plain and meets the Smith mountain and LeSueur section at the eastern margin of the sheet, on the flank of the escarpment of the upland plain. The details in the eastern part are essentially the same as in the section just mentioned. In the western half the granite field is crossed. At Knob Lick hill the porphyry forms the upper part, and the granite the lower.

Mine la Motte Section-The line of this section runs nearly east and west through the central part of Mine la Motte estate. The most of the sub-structure is coarse-grained granite, though Bald mountain is composed of porphyry. The basal sandstone appears in a small, basin-like depressions in the granite field. The porphyry mountain just alluded to protrudes above the sandstone, which is the typical LaMotte formation. At the edge of the limestone the basal sandstone is, as already stated and as shown by diamond drill holes, over 250 feet in thickness. The overlying limestone, Fredericktown formation, rests directly upon granite, in a knoll which is exposed at the surface

a short distance east of the main mines. The sandstone doubt

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FIGURE 18. Cross-section through the Mine la Motte Estate.

less surrounds the old hill, but its summit is higher than the top of the sandstone, thus bringing the limestones down to the crystallines. A fault of considerable throw occurs just beyond the east end of the section. The relations are shown in a similar way in the annexed cut (figure 18).

Knob Lick Section The cross-section which embraces Knob Lick hill, the center of numerous quarries, is instructive on account of being in the midst of the great granite field. The elevation is capped by a considerable thickness of porphyry which graduates downward into the coarse-grained granite. This is a type of a number of hills which exist in the central granite field.

Fredericktown Section-Although the extremities are four miles apart, the section under consideration is essentially a continuation of that of Knob Lick. It passes directly through the typical location of the Fredericktown limestone. It reaches from the outliers of the central granite field southeasterly to several isolated porphyry hills. At the bottom of the limestone, where this approaches the crystalline elevations, the basal sandstone and its associated conglomerate bed are very thin, bring. ing the calcareous member within a few feet of the massive rocks. The details of the succession and the arrangement of the several beds are especially well shown one mile north of the Little St. Francois river (figure 11 and also plate vi).

Devon Mountain Section-In direction the section is north and south. It passes through the limestone belt in an area where the crystallines protrude in numerous places. Devon and Buckner mountains are steep porphyry hills, with the Little