sandstone rests makes the practical demonstration of the exact position of the line in any particular place one of considerable uncertainity. There are but few outcrops showing well the relations of the two formations. One of the best is on the Little St. Francois river, just below the railroad bridge, north of Fredericktown. (See figure 11.) The details have already been given in another place. The ancient granite knoll has been partially uncovered through very recent erosion. The old surface is quite steep, and upon it rests the sandstone which, however, is soon replaced by limestone. Farther downstream is a similar exposure on the opposite side of the knoll.

Near the mouth of Rock creek, north of the Mine la Motte estate, the line of juncture is shown in the bed of the stream. For some distance the sandstone is nearly white and rests at one point upon the summit of an old granite hill which has just begun to be unearthed. The base of the sandstone is somewhat coarser than it is above and contains a few small crystalline bowlders. There are also several other points in the vicinity of Mine la Motte which show practically the same phenomena. Other good outcrops exist south of Knob Lick. The thickness of the sandstone varies greatly. Its present Surface clay.

4

7

13

2 Limestone, white, sandy upper surface is about 900 feet 10 Sandstone.black, yellow above the sea level, and con

12

and white.

Sandstone, white.

28 Sandstone, yellow.

178Sandstone, white
yellow beds.

21

Sandstone, pink. 8 9 Sandstone, reddish.

273A.

FIGURE 16.

and

Thickness and Variability of LaMotte Sandstone.

stitutes a part of the Farming. ton plain modified by recent erosion. The space occupied by it may be regarded as between this level and the uneven granite floor. It becomes attenuated over the old knolls, or thins out altogether against the larger Archæan hills; while in the ancient valleys it manifestly thickens very much. At Mine la Motte, a drill-hole (No. 211) put down at the western edge of Fredericktown limestone passed through 258 feet of this sand

stone. The dark red color of the rock at the bottom of the hole indicates that the drill had nearly reached the granite. The bed of limestone at the top, and the variability of the lithological characters of the sandstone are shown in the preceding cut (figure 16).

FREDERICKTOWN DOLOMITE.

The term Fredericktown is applied to the non cherty magnesian limestone, which forms a considerable portion of the surface of the Farmington lowland plain. It constitutes nearly all of the calcareous rock occurring within the area, and extends beyond the limits of the Mine la Motte sheet.

Two subdivisions rather distinctly marked are recogniz. able, the lower one being in the main an ordinary grayish limestone, with some sandy material and shaly layers, and occupying about one third of the total thickness, or about 75 feet; and the upper one, a buff dolomite, tolerably free from silicious matter, and having a maximum thickness of over 200 feet.

The lower limestone is the chief ore-bearing rock, and is typically developed at the Mine la Motte, where it is well exposed on account of the extensive mining operations carried on in that vicinity. By the miners it is further separated into an inferior or "white rock," and a superior or "black rock," each portion containing a distinct ore bed.

The white rock" is a light colored limestone, varying in places from white to gray or yellow, though some layers are often dark drab. It is more or less distinctly silicious, sometimes the silica being in finely divided state and sometimes in the form of sandy material. The oolitic beds occur locally. At the mine the "white rock" has an average thickness of about 50 feet. In the middle is the lower ore horizon, called the "bluff" bed, from its principal development at the Bluff diggings. It varies from 5 to 10 feet in thickness, the average being about 7 feet.

The "black rock" is a dark colored often shaly limestone. It is drab or bluish below and becomes lighter colored above, and has a total thickness of about 20 feet. Numerous thin

beds of dark shale occur with some sandy layers. The shales contain myriads of little fossils-the Lingulella lamborni ( Meek). The lower five or six feet make up the upper ore horizon, the "jack bed." It is distinguished from the lower ore stratum chiefly by the absence of silicious oolite and the presence of shale layers. The lithological details of the "black rock," as already given for the Rattlesnake diggings, section III, are as follows:

Details of Section at Rattlesnake Diggings.

3.

Limestone, thinly bedded, with clay partings, lighter colored than below.

.54

2. Limestone, heavily bedded, blue and gray, with tendency to slaty

structure......

1.

Limestone, same as No. 2, with galena (jack bed)

3.00 7.00

The location being near an old granite hill the characters as here exhibited probably do not extend very far, though they are similar at the Jack diggings on the opposite side of the crystalline mass.

Above the section at the Rattlesnake diggings the massive magnesian beds set in. These form the principal portion of the Fredericktown beds. As typically developed on the Little St. Francois river north of Fredericktown (figure 11), it is a buff to brown, compact, heavily bedded dolomite. Sometimes sandy beds are intercalated, but usually only when the calcareous strata approach the crystallines. As a rule, there is not much silicious matter. A chemical analysis of beds exposed on Rock creek north of Mine la Motte gave:

The proportion of lime and magnesia is very close to the theoretical amount for true dolomite.

As a part of the Fredericktown formation there occurs in the southwestern part of the sheet a few small areas of what are known as the marble beds. These are hard, compact, finegrained rocks, which are susceptible of taking a high polish. They occur in massive layers and may be taken out in large blocks. There is a wide range of colors displayed, from pure white, gray, yellowish, pink, reddish to dark red or brown. Often the various colors are blended, giving a mottled appearance to the rock. In the large area at the mouth of Cedar creek some rather unique phases are presented. These are best shown at the Allen and Smith quarry (figure 17).

10.

VIII. Section at Allen and Smith Marble Quarry.

Residuary deposits..

9. Marble, compact, red

8. Marble, with fragments of crystallines.

7. Marble, grading laterally into conglomerate

6. Marble, with large porphyry fragments.

5.

Conglomerate, coarse..

4. Shale, red, in part pebbly.

3. Conglomerate, coarse.....

2. Shale, red, with small bowlders

1. Shale, red, in part containing crystalline pebbles (exposed)

FEET.

4

8

5

8

113

4

212

8

3

10

The marbles appear to be locally altered limestones or dolomites. A complete change from one to the other often takes place in the same stratum within a distance of a few hundred feet. This transition is well exhibited just outside of the area at the Heberner quarry, on Marble creek (Tp. 32 N., R. VE, Sec. 17). The marble beds are continuous for a distance of perhaps 100 yards, but on each side of the quarry merge into ordinary limestones.

The geographical distribution of the Fredericktown limestone presents some noteworthy features. Within the limits of the sheet the formation occupies a broad curved belt, extending from the northwest corner eastwardly and southeastwardly to the eastern margin of the district and thence southwardly and southwestwardly. North of the sheet it is recognizable for a considerable distance, and southward, at least across Madison county. Farther than this its limits have not been traced with precision. It forms the eastern third of the Farmington lowland plain and is bordered on the east by the more or less pronounced escarpment formed by the higher cherty limestone of the LeSueur formation.

In the west the underlying LaMotte sandstone is the surface rock and cuts off the limestone in this direction. In a few places the crystallines form a portion of the boundary. At a number of points the ancient Archæan hills protrude above the limestone surface.

The geological position of the Fredericktown limestone is near the base of the Cambrian as represented in southeast Missouri, above the LaMotte sandstone, but below the LeSueur formation. However, in the area occupied by the Mine la Motte sheet there are often certain peculiarites in the distribution that prevents this stratigraphic sequence from being always made out clearly. The cherty limestone only overlies the noncherty rock in the extreme northeastern corner of the sheet. In some places, as in the vicinity of Doe Run, a similar somewhat cherty limestone rests directly upon the crystallines. Near Fredericktown only a few feet of sandstone intervenes between the limestone and the granites.