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with small fragments of porphyry. Some of the grits are very coarse, while immediately associated with them are very argillaceous shales. The shales are exposed in the railway cut just south of the big curve, at an altitude of about 1100 feet A. T., and immediately adjoining an outcrop of massive porphyry.

CORRELATION AND AGE.

A study of the sections and the other notes just presented, will reveal the fact that a detailed correlation of the members of the widely separated or even adjoining sections, is out of the question. We are forced to the conclusion that the minor differences of material, texture and color are local and extremely variable along the same horizon. A general classification of the rocks occurring within our area is, however, practicable, as has already been indicated. This classification is embodied in the following generallized section.

100' to 250'

20' to 200'

30' to 110'

10' to 160'

Beds of magnesian limestone with

upper portion.

As to the exact age of these Paleozoic rocks, with the exception of the evident fact that they are postAlgonkian, there is no evidence presented within the

intercalated sandstone strata in area of the sheet. Not a single fossil has been found here to the writer's knowledge. For the determination of their age, we are, therefore, dependent upon our interpretation of their stratigraphic relations to

Sandstone and grit, some lime

below.

stone, grading into conglomerate the rocks of surrounding
areas. Immediately east

Conglomerate; boulders of hema-
tite, porphyry, granite and mat- of the sheet, in Madison
rix of clay, grit or limestone.

Porphyry or granite, decomposed. St. Francois county, a few
Porphyry or granite, solid.

county, and north of it in

fossils have been found in rocks which we are satis

Fig. 8. General section of the Paleozoic rocks. fied belong to the same group as those included in the Iron Mountain sheet. The exact significance of these fossils, we regret to say, is not yet satisfactorily determined. Authorities differ as to whether the

rocks should be assigned to the Cambrian or to the Lower Silurian or Ordovician; the question must therefore be left open for the present. A discussion of the paleontological evidence would be out of place here, and we hence confine ourselves to this general statement of present conditions.

SOURCES OF MATERIAL.

The sources of the materials composing the Paleozoic strata of this region can be none other than the pre-existing Archean or Algonkian rocks around which the former were deposited. In the conglomerates and grits which overlie the solid and massive porphyry or granite, fragments of these older rocks are plainly recognized. In the sandstones which are above these, we find the quartz of the granites represented, and it is significant that the coarser sandstones of this region are found either in the immediate vicinity of a granite area, or in such position that the detritus from the decaying of these granites could be washed to the present sandstone localities.

In the arenaceous shales we have represented the smaller quartz grains of the granites, as well as the minute particles, or phenocrysts, of quartz which are recognized in the porphyries; the extremely fine texture of the sandstones near the base of the series in the vicinity of Pilot knob is most probably due to the fact of the smallness of the quartz grains or crystals in the surrounding porphyries. In the argillaceous shales we have represented the clay products from the decay of the feldspathic rocks; while in the great massive beds of limestone we have represented the calcareous and magnesian constituents which passed into solution and were redeposited through chemical or organic agencies. How great the time, how vast the amount of decay of these firm and resistant rocks in order that this great volume of sedimentary material could be accumulated, cannot be readily pictured.

DYNAMICAL GEOLOGY.

Structure. The Paleozoic rocks of the Iron Mountain sheet present very little evidence of disturbance or movement. The beds within the larger areas, away from the Archean hills, generally lie in a hori zontal position; slight flexures are, however, apparent at times, but these are not persistent. Around the sides of the Archean hills the rocks invariably dip away from such centers, and this has been attri

buted by some to a vertical movement of the older crystalline rocks. This conclusion does not seem, however, to be well founded; the phenomenon has all the appearance of being due principally, if not entirely, to the fact that the beds were deposited on the sloping floor of the submerged hill. Under these conditions they would naturally slope away from the hill as a center. The invariableness of these conditions corroborates the conclusions herein expressed, as does also the fact that the strata taper out successively up the slope of the hill. It is hardly possible to conceive of these hills having been moved vertically upward without the surrounding rocks being disturbed, and thus the horizontality of the strata in the valleys is against any such movement having taken place. Therefore, the subject may be dismissed with the remark that there is only such flexing or irregularity in the attitude of these rocks as can be accounted for by the inequalities of the floor upon which they were laid down, or by the settling of the rocks after their deposition, supplemented perhaps by a gentle oscillation during the regional uplift which brought these rocks above the surface of the water.

Sub-areal Decay and Original Thickness.-The great sub-areal decay of the rocks of this region has already been treated of in this report. The fact that remnants of the Paleozoic rocks are found within the area, at an altitude of nearly 1600 feet (see page 28), whereas at other points their limits are as low as 1000 feet A. T., indicates what a vast amount of rock has been removed. The products or residues of this decay are clay, sand, chert in nodules and large blocks, and indurated sandstone. Over a large part of the country, as has already been referred to, there are great accumulations of chert nodules, which have given to the hills covered by them the name of "Flint hills.” Over portions protected from vigorous erosion, accumulations of residuary clay exist and are encountered in excavations. In the railway cut at Tiptop, about five miles south of Ironton, a great body of such clay is exposed to a thickness of 30 feet or more. It is made up of different materials; part is apparently of clay proper, a large portion of which is of a yellow and drab color, while along the seams it is of a white color, like true kaolin. An analysis of a specimen of this clay showed, however, that it is composed in large part of silica, even the white kaolin-like portion containing over 60 per cent. The following results show its composition:

[blocks in formation]

A large part of the deposit is sand, which occurs in strata-like beds dipping at a high angle to the south, and, further, blocks and fragments. of white quartzite and nodules of quartz are included in this mass.

It is impossible at the present writing to fix the exact upper limit reached by these sedimentary beds. They probably extended somewhat above the highest limit noted, but we are inclined to think that they did not go very far beyond this. Our principal reason for this conclusion is the fact of the entire absence of any of their remains overlying any of the porphyry hills which reach the altitude of 1800 ft. A. T. Of course, these summits have been subjected to vigorous erosive action; but yet we are inclined to think that, if the cherty upper members of the series had ever covered such hills, some of this almost indestructible chert would remain to bear witness of the fact. As we go farther west, toward the limit of the Archean area, such remnants are found on the flat tops of the porphyry hills, even though the sides are destitute of them.

Metamorphism.-Strictly speaking, we are not inclined to class these strata among metamorphosed rocks, in the sense that they have been altered or indurated by an excessive amount of heat. There is entire absence of any evidence that great heat has anything to do with their present condition. There have been no igneous rocks ejected in their vicinity since their formation; there have been no violent or great orographic movements; they have never been buried deep enough beneath superincumbent strata for subterranean heat to have altered them. Their present highly crystalline or indurated condition is, in the writer's opinion, due almost entirely to the action of percolating waters, which also could have affected their dolomization; the open, porous structure of many of the massive beds would be a natural result of the reduction in volume caused by the substitution of carbonate of magnesia for carbonate of lime.

The marbles of this and adjoining areas have been adduced by some as additional evidence of metamorphic action. As has already been intimated on page 30, we do not consider them necessarily such. These marbles are not truly crystallized limestones, composed of welldefined and separate crystals; they are more properly dense crystalline limestones, variously tinted and susceptible of polish. The differences in structures and properties between these rocks and the surrounding dolomites may well be attributed to the difference in composition shown on page 28.

ECONOMIC GEOLOGY.

The minerals of greatest economic value within the limits of the sheet are iron ores and building stones. Lead ores are mined near the northeastern corner, but, though the town of Doe Run is within the boundary lines, the main deposit of ore is just beyond the limits, and will be more appropriately described in connection with the deposits of Mine LaMotte of the adjoining sheet. Clays, limes and sands of value for ordinary local uses are also found and will receive brief treatment.

THE IRON ORES.

By Frank L. Nason.

The St. Francois mountains have a world-wide reputation as producers of iron ores; yet, notwithstanding the great areal extent of the porphyry and granite rocks which make up the mass of the mountains, their reputation depends principally upon two localities. These two localities are at Iron Mountain and Pilot Knob. There are other specu lar ore deposits in the vicinity of these two great deposits, but these sink into insignificance in comparison with the first named.

1

Iron Mountain has produced, in round numbers, 3,500,000 tons; Pilot Knob, about 1,500,000 tons. The amounts of ore produced at other localities are as follows: Shepherd mountain, 75,000 tons; Cedar mountain, 25,000 tons; Buford mountain, 3000 tons; Russell mountain, 3000 tons; Shut-In, 600 tons. This gives a total of 5,106,600 tons. Of this amount Iron Mountain has produced 68%; Pilot Knob 29%;

1 The records at Pilot Knob prior to 1882 were destroyed by fire. From 1882 to 1892 inclusive, Pilot Knob has produced 1,074,000 tons. This and much of the data in the following descriptions have been obtained through the kindness of Prof. W. B. Potter, of St. Louis, the engineer of the Iron Mountain company.

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