xt7tmp4vj37s https://exploreuk.uky.edu/dips/xt7tmp4vj37s/data/mets.xml Moore, Philip North, b. 1849. 1876  books b96-12-34873188 English Printed for the Survey by J.P. Morgan & Co., : [Frankfort, Ky. : Contact the Special Collections Research Center for information regarding rights and use of this collection. Iron ores Kentucky. Report on the iron ores of Greenup, Boyd & Carter counties  : the Kentucky division of the Hanging Rock iron region / by P.N. Moore. text Report on the iron ores of Greenup, Boyd & Carter counties  : the Kentucky division of the Hanging Rock iron region / by P.N. Moore. 1876 2002 true xt7tmp4vj37s section xt7tmp4vj37s 

         N. S. SHALER, DIRECTOR.






            BY P. N. MOORE.


59 & 60


 This page in the original text is blank.



  The iron ores of this region belong to the class of earthy
carbonates or siderites, known also as clay iron-stone, and lim-
onites or hydrated peroxides, which result from the alteration
and oxidation of the carbonates. They are all in what may be
properly called stratified deposits; for although some of them
do not form connected strata, they all occupy regular, well-
defined geological levels, which they hold over wide areas.
  They are all found in association with the rocks of the coal
measures, beginning at the base with an ore resting on the
sub-carboniferous limestone.
  There are occasional scattering beds of nodular ore in the
shales of the Waverly formation; but they are never extensive
enough in this portion of the State to be of any value.
  In Kentucky, as in many other States, the ores occur in
greatest abundance in the lower coal measures. There is no
ore of more than local importance above this level. Some
are occasionally found in the middle coal measures, and will
be described hereafter; but they are by comparison of very
little value, and are not largely worked, as they are inferior in
quality and untrustworthy in occurrence.
  The usual method of occurrence of the ores of this region
is in beds or layers, although there are exceptions to this which
will be described. At the outcrop, on the hillsides, where the
ore has been exposed to the action of the atmosphere, it is
usually a limonite; but as it is followed into the hill where it has
been protected by a heavy covering, it occurs as a carbonate or
siderite. This is always found to be the case where the ore
has been protected from atmospheric influence.
  The distance from the outcrop at which the siderite will be
found, varies of course with the character of the overlying rock.
Where this is a porous sandstone, the limonite will be found
                                                           6 i



extending a long distance under ground; and if it be near the
top of a hill, the whole bed is sometimes found changed to lim-
onite. If a dense, impervious clay shale overlies the ore, the
siderite usually extends very nearly to the outcrop, and only a
very small band or rim of limonite surrounds the bed.
  Where the ore is covered by water for the most of the time,
and thus protected from the air, it is usually found as the car-
bonate. This feature has been noticed at a number of places.
Where it is comparatively dry the ore is usually a limonite;
but when in a ravine, for instance, where water is constantly
percolating through the rocks, keeping them saturated, the
carbonate is almost always found.
  These facts would seem to indicate that atmospheric influ-
ences alone effect the change from carbonate to hydrated per-
oxide of iron. There are, however, other facts which will be
referred to further on, which seem to indicate that it cannot
have been altogether effected by this means; and in spite of
the seeming contradiction, it must have been assisted by the
action of water.
  In every instance the limonite has been derived from the
carbonate; and in no case whatever has any indication or trace
of a change by deoxidation from peroxide to carbonate been
observed. The siderite or carbonate of protoxide of iron is
the original mineral of these ore beds.
  The siderites possess a dense, close structure, with the sili-
cious and other impurities disseminated evenly through the
whole mass, and a specific gravity which is much higher in pro-
portion to the per centage of iron than it is in the limonites.
This singular fact has been before stated by Andrews in the
Ohio Geological Report for 1870, page 214. The limonites
show an open, porous structure, with layers of different qualities
of ore, and the impurities often segregated to themselves in
the center of the specimen. They contain on an average a
considerably higher per centage of iron than the siderites, and
are much more esteemed by the iron manufacturers of this
region. In fact, at the majority of the charcoal furnaces of
this region, nothing but the limonite ores are used, as the



managers profess not to be able to produce a coarse-grained
foundry iron from the blue carbonate ores. This is generally
supposed to be owing to the presence of sulphur. Analyses
show that there is usually somewhat more sulphur in the car-
bonates than in the limonites, which may be sufficient to ac-
count for the trouble experienced in working them; but it is
also probable that the dense close structure, rendering them
difficult of reduction, has much to do with the matter.
  The average from a large number of analyses made by Dr.
Peter and Mr. Talbutt of ores from this region, gives the rela-
tive per centages of metallic iron in the limonites and carbon-
ates as follows:

           OP ES.               Liwonites.       Carbonates.

Limestone ores.                    46.22             33.60
Upper block ores.44.60                                  34.42
Lower Wlo.k ores.                  33.50             29.73

  It will be seen from the above that the average per centage
of iron in the limonites is considerably greater than in the car-
bonates. The specific gravity of the carbonates, as already
stated, is higher, in proportion to the amount of iron present.
than that of the limonites.
  We thus have the singular fact presented of the carbonate
ores gaining in the per centage of iron in the change to limon-
ite, while the specific gravity, which is commonly supposed to
be somewhat proportional to the per centage of iron, becomes
considerably less. This is a fact which is not thoroughly un-
derstood and appreciated by the purchasers of ore in this region.
They do not realize that the carbonate ores will not yield as
much in proportion to their weight as the limonites, and that
it is poor economy to pay the same price per ton for both.
                   THEORY OF FORMATION.
  It is generally agreed by geologists that these ores are
deposits from aqueous solution, laid down by chemical action.
  There seems to be no doubt of this whatever. There are.
however, different theories held in regard to the method and




time of this precipitation, and as to the condition of the ore
when first deposited. The feature for which it is most difficult
to account, is the occurrence of the ores as carbonates.
  Under the circumstances at present prevailing, we should
expect the ore to be precipitated from solution as a peroxide.
  The waters which originally held the iron in solution must
have been of wide extent, for we find some of the ores extend-
ing over large areas, sometimes covering hundreds of square
miles; but whether these waters were oceanic in character,
or large, shallow, fresh water lakes, is not well settled. It
seems probable, from the occurrence of certain plant remains
which are occasionally found in the ores, that the waters were
shallow and fresh; but, on the other hand, there are occa-
sional organic remains which are apparently of marine origin.
Whence also the oxide of iron was derived, which the waters
afterwards spread in so even a coating over the bottom, must
be left to conjecture. It is a problem of no small difficulty to
account for its so general dissemination and subsequent regu-
lar deposition. One theory holds, that the ore beds have all
been deposited originally as peroxide of iron, and subse-
quently changed to carbonates, through the agency of the
carbonaceous matte-r of the interstratified coal beds, and that
disseminated through the associated shales and clays.
  This theory has in its favor the fact that it is based upon
a precipitation of the iron in the condition which we should
reasonably expect it to assume; and there is no doubt that
carbonaceous matter is competent to effect the change from
peroxide to carbonate; but whether it would do so in such great
masses, and over so wide a field, is a matter of considerable
doubt. Moreover, this theory requires a constant association
of carbonaceous rocks with the ores such as we do not find
existing in nature. We often find the ores separated by wide
intervals from any rocks which contain a perceptible amount of
carbon-intervals so great that no possible effect can have
been exerted. It is not probable that the carbon of, or the
carbonated gases emanating from, a coal situated at a distance



above the ore, can have had any influence upon it; and this is
the relative position they often occupy.
  It is also extremely probable, or almost certain, if the
change had been effected in this way from peroxide to carbon-
ate, that it would not have been perfect in every instance;
some local interruption would have interfered, and we should
sometimes find places where the ore was in its original con-
dition, or only partially changed to the carbonate. No such
specimen, showing an alteration from peroxide to carbonate,
has ever been found in this region by any member of the Sur-
vey, but in every instance it is found as the unaltered carbon-
ate, or showing change from carbonate to peroxide.
  Another theory is, that the ore beds are continental rock,
pseudomorphs of carbonate of iron after limestone, or, in other
words, that all these deposits were once beds of limestone-
now changed to iron ore. It supposes that the iron was dis-
seminated through the shales which had been deposited as a
ferruginous mud above a bed of limestone. By a process of
segregation, probably acting through the agency of carbonated
waters, the ore has been separated and carried down upon the
limestone, which in its turn was dissolved and carried away.
  We should expect, however, under the conditions most prev-
alent at present, that the iron would be precipitated as the
peroxide; but Bischof has shown that carbonate of iron is
pseudomorphous after carbonate of lime, and that it is depos-
ited where free access of air is prevented through an excess
of carbonic acid or other cause. Either the exclusion of the
air or the presence of an atmosphere largely composed of car-
bonic acid must, therefore, have been the condition which
enabled the deposition of the iron as carbonate.
  This theory explains very satisfactorily the manner of occur-
rence of a certain large class of the ores of this region, known
as the " limestone ores," as will be shown more fully hereafter;
but it is improbable that the others can have been formed in
this way. If all were thus deposited, we should expect to find
them frequently changing to beds of limestone; but the fact
Chemical and Physical Geology by G. Bischof-English edition, vol. 11, 51.
    VOL. 1-5                                               65




is that certain of the most regular and persistent strata of ore
in this region have never yet been traced to a replacement of
this kind. They are occasionally found fossiliferous, and some-
times very calcareous; but they always maintain their identity
as ore beds, and hold a uniformity of character and thickness
such as the limestone ores do not possess. They seem to have
been deposited as layers or beds of carbonate of iron in the
shape and place we now find them, and before the overlying
rocks were laid down. It is a matter of some difficulty to con-
ceive of the conditions which would allow of the deposition as
carbonate instead of peroxide over so wide an area; but there
seems to be little doubt that it was so deposited.
  Still another class, known as the nodular or kidney ores,
seem to have been mixed with a great deal of earthy matter,
and deposited as a fine ferruginous mud in quiet, fresh water;
the iron afterwards segregating into the nodular form, and the
earthy matter forming clays and shales.
  We thus see that these ores vary considerably in their man-
ner of deposition, while retaining the same general character.
  This subject will be referred to again in discussing the gen-
eral divisions of the ores.
  In the subsequent alteration of these ores, the change from
carbonate to limonite, which is even now going on, there are
two agencies which seem to share in the work-air and car-
bonated waters.
  The evidence of the effect of atmospheric influence in this
alteration has been already stated. It is one of the most
powerful agencies; but there are some of the ores which have
undergone a rearrangement, such as cannot have been effected
by this alone. It must be the result of a solution, more or
less complete, and a redeposition in nearly the same place in
another chemical and molecular condition. The agent which
has accomplished this is probably the same that has so often
borne an important part in the original deposition, namely:
water charged with carbonic acid. The change which is alluded
 For a fuller (liNcUSO.iln of the different theories in regard to the formation of the carbonate
ores, see Lesley's Iron Manufacturer's Guide, page 633 et seq.



to is shown by many of the ores, but most perfectly by certain
of the upper block ores. It is the concentric arrangement of
the layers of limonite, which are often fibrous and nearly pure,
and the segregation toward the center of the silicious and
earthy matter, which was evenly disseminated through the
whole specimen before the alteration was begun. Specimens
are frequent showing the change at all stages.
  It seems to have taken place from the outside. The car-
bonated water gaining access to a specimen on all sides by
means of the cracks and seams which abound in the ore, dis-
solves a portion of the carbonate of iron, which, not being pro-
tected from the influence of the air, is changed on the escape
of the free carbonic acid to a peroxide, and deposited at once
as a layer of limonite surrounding the center of carbonate.
The carbonated water dissolves readily, carbonate of iron,
lime, and magnesia, but has little influence upon alumina and
silica. The iron is precipitated, the lime and magnesia are
mostly carried off in solution, but the silica and alumina not
being affected, are gradually separated toward the center, being
surrounded by successive layers of limonite, until finally the
iron of the whole specimen is changed to limonite, and we
have a hollow ball of ore, in the center of which is a mass of
silicious clay surrounded by successive layers of limonite.
Of course, during the progress of this change, before it is com-
plete, the carbonate of iron in the center grows more and
more silicious. Many specimens have been found showing
this to be the case, and showing the successive increase in the
proportion of the silicious matter in the center as the change
is nearer completed. The analyses of many of the ores in
which these features are most common show a decidedly less
per centage of lime and magnesia and a much greater per
centage of metallic iron in the limonites than in the carbonates,
thus corroborating this theory of the method of alteration.
This process is of the greatest benefit to the ore; by it the
per centage of iron is materially increased, sulphur, lime, and
magnesia partially removed, and the silicious impurities sepa-
rated in such a way, that, after calcining, they can be largely




removed by screening. It is much more complete than any
artificial process. It has the disadvantage, however, of ren-
dering the ore brittle and shelly, so that it crumbles to pieces
easily after calcining, and a good deal of it is wasted as fine
ore and dust.
  All of the ores are not altered in this way. Many of them
seem to have been acted upon by the air alone, or only slightly
assisted by other causes. These show a simple oxidation;
sometimes in concentric layers; but their structure is quite
homogeneous, and the impurities remain disseminated through
the whole mass.
  They show a less increase in the per centage of iron, and
no marked removal of impurities in the change from carbonate
to limonite. The lower block ores are of this class more gen-
erally than otherwise, and, as shown already by the average of
a number of analyses, the increase in per centage of iron in
these is only about half that in some of the other ores.



  The ores of the region under consideration, which all belong
to the same general mineralogical class and geological position,
differ considerably, as already shown, in their external appear-
ance, chemical composition, and the circumstances of deposition.
  Three general divisions of them are recognized, and will be
described under the names commonly applied to them. They
             (a) The limestone ores.
             (b) The block ores.
             (c) The kidney ores.
  These names are often arbitrarily and incorrectly applied;
but they are sufficiently accurate and comprehensive for our
purpose, and they possess the additional advantage, that, in
their sequence, they approximately represent the geological
position, as well as the external appearance.
  The limestone ores are so called from their association with
limestone, being usually found resting upon it.
  The block ores are named from the peculiarity that they
possess of cleaving into sharp, square-cornered blocks, as they
are raised by the miners.
  The kidney ores are so called from the shape which they
usually assume.
                 (a) THE LIMESTONE ORES.
  Properly speaking, the name limestone ore is applicable only
to an ore which is deposited upon, or very near to, a limestone;
but often the ore occurs at the same geological level, in the
same position, with reference to the overlying and underlying
rocks, over a field wider than the associate limestone. In this
case the name is still given to the ore.
  Owing to their comparative purity, uniform character, rich-
ness in iron, and the ease with which they are worked in the



furnace, these ores are valued more highly by the furnace men
of this region than either of the others. They occur both as
limonites and unaltered carbonates (or siderites), and each of
these shows modifications which are rare in other ores.
  The limonites of this division are found (I, and rarely) as
lightish brown, semi-concretionary ochreous ores, and (2, more
commonly) as a dense, dark red, close-grained ore, giving red
streak and powder, but containing about the same per centage
of combined water as the other. When of this character, it is
often full of seams and crevices, which have been filled with
calcite, and it is apt to adhere to the underlying limestone.
This variety is known as red limestone ore, and is the most
valuable of any found in this region.
  The following analyses will serve to show the chemical com-
position of this variety of ore. In some of the specimens the
process of oxidation has not been completed, and there re-
mains a considerable proportion of carbonate of iron; but the
samples were taken to represent the ore as fairly as possible,
as it is actually mined and paid for by the furnace operators.
  These analyses, as well as all the following, unless otherwise
specified, were made by Dr. Peter and Mr. J. H. Talbutt,
chemists of the Survey.

               TABLE 1.-LIMESTONE ORES-Limonites.

                 ____2I3      4   5    6   7    8   9    l    I

Peroxide of  i ron. 6o..576 67.859 46.984 72.95  49.770 81.64So.o4o 57- 55s. 80: 71  6o8r 6
Carbonateof iron.. 15-56,37            h9 . . . . . . . . 5 .
Alomin..2 .860           S. C 5.5.631 53.,601..
frownoxide of mangane e....98oj.....64..6 40   t e   t. 1  .30.       . .9 .
Carbonat e of lim..e... . .   . e.. . .. . .38.380....... ..... .      ..  c
Carboaeolie.-..f.-........ trc. .  - 2  -- e-4-  &. 38-  3.1  -1  t rce - -5- 7. 48  . 38s.  -is
Crbonate of magnia...6.91- 752.g..8.1 .15 .99  .425.758     .04-. 35 1
Phosphoeacd...... 632 .143 .371     .3   6o. .115.057 .57 .o84 .a61
Siulphric acid . notest.notei.notet. .  1781no1et.lnotest .64 ...5 .o89 27.
Silica andinsolubleslicates  o.6;  15.  . 7 6- 33.2  2.6o  6.1 05.45 15 7  1s 6  05 930
Combied .ater . . . . 7. 14 2.93 26.599  8.452  9.  ".801 _o.o  1.390 87721 . 21 .141
Total. . .   .  .   .oo.Oooloo.  99.6.6 99. 839 loo-. o84-. 58 -oo  -0- . X 0.co

Metallic iron . ............49-995 47.501 39.025 51.07  34-739 56.248 s6._28 40.285 41.357 50.176 4.44
Suiphor........                     .        7 .042.035 -7.          .341
Ph-ophoeos..76.o....           ..  ..234.2 .     -24 .3  .0-36.0.70
   And loss.
   No. i. Sample of limestone ore from Hood's Run, branch of
Tygert Creek. Ore used at Raccoon Furnace.

1 2


No. 2. Limestone ore from head of Two Lick Creek, Kenton
No. 3. Limestone ore from Coon Fork, Kenton Furnace.
No. 4. Limestone ore from Shover Drift, Kenton Furnace.
No. 5. Limestone ore from Powder Mill Hollow, Kenton
No. 6. Limestone ore, cabinet specimen, best quality, Boone
No. 7. Limestone ore, cabinet specimen, picked to represent
the best by Mr. A. R. Crandall, Assistant of the Survey, from
land of S. Warnock, Tygert Creek.
  No. 8. So-called slate ore, occupying the place of the lime-
stone ore, ridge between Cane Creek and Wilson Creek,
Hunnewell Furnace.
  No. 9. Limestone ore from Hood's Creek, Bellefont Furnace.
  No. io. Limestone ore from the Graham bank, near Willard,
Carter county; average taken from the stock bank at Willard.
  No. I I. Limestone ore from Brush Creek, Pennsylvania Fur-
nace property.
  All of the above analyses, with the exception of Nos. 6 and
7, were made from average samples taken by myself.
  It will be seen that in the average samples the per centage
of iron varies from 34. to 5I. Silica, alumina, lime, and mag-
nesia are present in exceedingly varied proportions. Silica
and alumina are always present in appreciable quantities, but
the lime and magnesia vary exceedingly. The amount of sul-
phur is small; so small as to have little or no influence upon
the ore, unless, perhaps, in the case of No. I I.
The amount of phosphorus varies widely, and in some ores
is present in sufficient quantities to make a " cold-short" iron.
The unaltered carbonates or earthy siderites occur (I) as
amorphous, dense, close-grained ore, varying in color from
light brown to dark blue, and commonly known as " blue lime-
stone ore," and (2) as a light-colored, coarse-grained oolitic ore
called "grey limestone ore." It consists of grains of siderite,
embedded in a light-colored silicious matrix. It is more com-
monly found associated with the ferriferous than with the sub-




carboniferous limestone. It is highly valued and much used;
but it is apt to become suddenly poor in iron and very calca-
reous, and the character of the ore is such that this change can-
not be readily detected by the eye.
  The following table of analyses will show the composition of
some of the limestone ores, siderites, of this region:

               TABLE 11.-LIMESTONE ORES-Siderites.

                       2I  1  2       3      4       5       6

Peroxide of iron ......  4.410 I 5-945  27.296  26.240   12.784   31-544
Carbonate of iron ... .  61.220  65.018  44.242  27.511  32.285   30.708
Alumina.... . .. .    2. 260  i.o6o  1.560   9.021  1i.968   1 779
Carbonate of manganese...150  2.332   .842    .270    .465    .o60
Carbonate of lim.e.       4.480   2.720  6.580   2.320  21.125   2.730
Carbonate of magnesia..  trace.  9.038   1.o46  2.838    .691    .144
Phosphoric acid......    .313    .255   .732    .499    .377    .421
Sulphuric acid......  . not est.1.280  4.587    .i6     .267    .491
Siica and insoluble slicates.2i.260  10. 260  i1 . l60  25.180  19-730  25.430
Combined water and loss.5.367 2.112  1.955   6.oo5    .308   6.523
Potash..... .. . ..    .231
Soda.        ..... . .. ..     309
Total... .. . .. .    .      .    ioo. oooloo.ooo loo.ooo ioo.ooo

Metallic iron ..... . . .  32-577  35 -549  40.465  31-598  24.591  36.627
Sulphur.     .      .          533    1.855   .046    .107    . 196
Phosphorus..           .136    .111   .319    .217    .164    .184

  No. I. Limestone ore from        Old Orchard diggings, Boone
Furnace, Carter county.       Average sample from the whole bed
  NO. 2. Ore from Horsley bank, Boone Furnace. Analysis of
a cabinet specimen of the undecomposed carbonate,
  No. 3 Average sample from the ore at Horsley bank, Boone
Furnace. Only a comparatively small amount of ore was ac-
cessible at this point, and it is not unlikely that it may have
been unusually sulphurous-more so than the general average
of the bed.
  No. 4. Blue limestone ore from bank of Tygert Creek, about
two miles above Iron Hills Furnace, Carter county.
  No. 5. Grey limestone ore from J. P. Jones's drift, near
Ashland, Boyd county. Average sample; ore at this place
much more calcareous than usual.



  No. 6. Grey limestone ore from Mt. Savage Furnace, Carter
county. Average taken from ore at the stock pile by Mr. J.
A. Monroe, aid of the Survey.
  These analyses show a much greater and more constant
proportion of lime and magnesia than in the limonites. Sul-
phur and phosphorus are also somewhat larger.

                   GEOLOGICAL POSITION.
  The limestone ores occur at several well-defined levels; but
there are two beds of greatest importance. The first is also
the lowest ore of this region. It rests upon the sub-carbonif-
erous limestone, at the base of the coal measures of eastern
Kentucky. The other ore rests upon a limestone which has
been called the ferriferous limestone by Andrews in the Ohio
Geological Reports-a name which will be retained. In addi-
tion there are several small beds of minor importance which
occur above the ferriferous limestone, but they are of local
range, and have small value compared with the others. They
will be referred to hereafter. The ore of the sub-carbonif-
erous limestone will be known as the lower limestone ore; that
of the ferriferous limestone as the upper or ferriferous lime-
stone ore. The special description of each of these beds will
be given hereafter; but this much of a description has been
given here, as it is necessary in order to properly discuss the
geographical range of these ores, and to the correct under-
standing of the position of the other ores to be described.
  The limestone ores present a greater variation in thickness,
and more sudden changes, within a short distance, than any
other ore of this region. They vary in thickness from a few
inches to as many feet; but the rule usually holds, that the
thicker the An pocket " or "d roll," the less is its horizontal extent.
  There are, however, marked exceptions to this, as, for in-
stance, at Boone Furnace, in Carter county, where, at the head
of Grassy Creek, on the ridge forming the divide between the
waters of Tygert and Kinnikenick Creeks, the limestone ore
occurs very regularly from fifteen inches to two feet in thick-
ness, and often much thicker, in wide pockets. At the Graham

I 5



bank, near Willard, Carter county, this ore is found in the usual
rolls, some of which measure two to four feet in thickness.
  The limestone upon which the ore is deposited presents an
exceedingly uneven surface, being full of ridges and depres-
  Into them the ore seems to have settled in greatest quantity,
growing thicker over the depressions in the limestone. and
thinner over the ridges, until the top of the ore often presents
a series of ridges and hollows which are the reverse of those
in the limestone. The following sketch will show this feature:

  At some places it might be supposed that the ore had flown
into the pre-existing irregularities of the limestone in the
plastic condition of a recent chemical precipitate. This sup-
position might explain the fact if we found the top of the ore
presenting an even surface; but this is rarely if ever the case.
It is usually uneven and irregular, as shown in the cut above.
  This supposition is therefore probably untenable. The more
probable theory of formation of the limestone ores is, that they
have been formed, in most cases, by a segregation of the iron
from the shales and clays above the limestone after their depo-
sition. The iron in the form of a carbonate, held in solution
by strongly carbonated waters, has been carried down and
deposited upon the limestone, which was partially dissolved
and carried off by the waters which deposited the iron, thus
forming the depressions and irregularities in its surface which
have been already described.
  This theory of deposition by segregation from the overlying
rocks accounts most fully for the following marked peculiar-
ities of the limestone ores; the irregularities of thickness; the
tendency to become suddenly calcareous, or to disappear alto-
gether, giving place to a limestone; the comparative freedom
from coarse silicious impurity, and the fact that an increase


in thickness is not, as in the block ores, accompanied with a
corresponding deterioration in quality and increase in the pro-
portion of sand intermixed,
  It furthermore explains the presence of the thick beds of
fire-clay in which barely a trace of iron remains, which are so
often found above these limestone ores. Marked examples of
this occur at Amanda Furnace and on Pea Ridge, between
Hunnewell and Pennsylvania Furnaces.
  The most common impurities of these ores, which serve to
lessen their value at places, are chert (or flint) and lime.
  The chert is more commonly associated with the lower lime-
stone ore, which rests upon the sub-carboniferous limestone,
than with the ore of the ferriferous limestone. Where present,
it is of great injury to the ore; for a very small per centage of
chert interferes seriously with the easy working of the ore in
the furnace. Lime, on the other hand, is of no injury, save
that, by acting as a diluent, it reduces the per centage of iron.
The variation in the per centage of lime present is very great,
and the change from a rich ore to one more calcareous, from
a calcareous ore to a ferruginous limestone, takes place very
suddenly. This change is more apt to occur in the siderites
than in the limonites; for the agents which have effected the
alteration of the ore seem to have also removed the greater
proportion of the lime present.
  Much of the low-grade calcareous ore, yielding from fifteen
to twenty-five per cent. of iron, is now refused by the furnace
managers at any price, for the reason that, although they know
it could be used profitably as a substitute for limestone as flux,
they have no cheap and ready means of determining its value.
Another reason is, that the ores vary so suddenly in composi-
tion that the proper proportion to be used in the furnace
charges would be constantly subject to change. Their value
can only be accurately determined by often repeated chemical
analyses; and as yet there are no facilities in this region for
having such work cheaply done.
There are large quantities of these low-grade limestone
ores, or ferruginous limestone