GEOLOGICAL SURVEY CIRCULAR 633 

Standards for the 

Classification of 

Public Coal Lands 

Abstract -----------------------------------------------------------------------

Introduction ---------------------------------------------------------------

History -----------------------------------------------------------------------

Purpose of classification ---------------------------------- _______ _ 

Present standards----------------------------------------------- _______ _ 

Maximum depth ------------------------------------------ _______ _ 

Maximum depth for coals of 

minimum thickness ________________________________________ _ 

Use of as-received Btu values ________________________ _ 

Minimum heat value ________________________________________ _ 

Minimum thickness ____________________________________________ _ 

Maximum thickness ----------------------------------- _______ _ 

Thickness of coal beds containing partings_ 

CONTENTS 

Page 

1 

1 

1 

2 

3 

5 

5 

5 

5 

5 

6 

6 

Present standards-Continued 

Treatment of coal beds of 

irregular thickness ----------------------------------------

Treatment of a thin coal associated 

with a thick coal and treatment of a group of coal beds _______________________________ --------

Burned coal _____________ ---------------------------------------------------

Quality -------------------------------------------------------------------------

Diagram (fig. 1) showing depth limits of coal~---

Areal basis for classification ___________________________________ _ 

Summary of the standards for the 

classification of public coal lands ______________________ _ 

References cited _______ --------------------------------------------------

ILLUSTRATIONS 

FIGURE 1. Diagram showing depth limits of coals of different Btu (as-received basis) and of different 

Page 

6 

7 

8 

8 

8 

9 

9 

10 

Page 

thicknesses less than 6 feet____________________________________________________________________________________________________________________________ 4 

2. Diagram showing split or broken beds and their value for classification________________________________________________ 7 

III 

Standards for the Classification of Public Coal Lands 

By N. Wood Bass, Henry L. Smith, and George H. Horn 

ABSTRACT 

In order to provide uniformity in the classification of 

coal lands in the public domain, certain standards have 

been prepared from time to time by the U.S. Geological 

Survey. The controlling factors are the depth, quality, 

and thickness of the coal beds. The first regulations 

were issued April 8, 1907; others followed in 1908, 1909, 

and 1913. Except for minor changes in 1959, the regulations of 1913, which were described in U.S. Geological 

Survey Bulletin 537, have been the guiding principles 

for coal-land classification. Changes made herein from 

the standards previously used are: ( 1) a maximum. 

depth of 6,000 feet instead of 5,000 feet, (2) a maximum 

depth of 1,000 feet instead of 500 feet for coals of minimum thickness, (3) use _of Btu (British thermal unit) 

values for as-received coal instead of air-dried and 

(4) a minimum Btu value'.of 4,000 for as-received coal 

instead of 8,000 for air-dried. An additional modification is that the maximum thickness of 8 feet which was 

designated in the Classification Chart for Coal Lands 

in 1959 is changed to 6 feet. The effect of these changes 

will be the classification of a greater amount of the 

withdrawn land as coal land than was done under 

earlier regulations. 

INTRODUCTION 

The purpose of this report is to set forth certain changes in the standards for determining 

what lands in the public domain are classifiable 

as coal lands. The controlling factors in determining what lands are coal lands are the depth, 

quality, and thickness of the coal beds. Changes 

made from the standards previously used 

(Smith and others, 1913) are: (1) a maximum 

depth of 6,000 feet instead of 5,000 feet, (2) a 

maximum depth of 1,000 feet instead of 500 feet 

for coals of minimum thickness, ( 3) use of Btu 

(British thermal unit) values for as-received coal 

instead of air-dried, and ( 4) a minimum Btu 

value of 4,000 on an as-received basis instead of 

8,000 on an air-dried basis. An additional modification is that the maximum thickness of 8 feet 

which was used in a Classification Chart for 

Coal Lands (U.S. Geological Survey unpub. 

chart, approved by Acting Director, Sept. 9, 

1 

1959) is changed to 6 feet. The effect of these 

changes will be the classification of a greater 

amount of the withdrawn land as coal land than 

was done under earlier regulations. A minimum 

thickness of 14 inches for coals yielding 12,000 

Btu or more, which has been in effect for almost 

60 years, is not changed. The minimum thickness 

for coals of lesser Btu values is increased at 

specified rates. 

Many geologists of the U.S. Geolor,-ical Survey have contributed to the preparation of this 

report. 

HISTORYl 

Prior to May 20, 1862, when the Pomestead 

Act was passed by Congress, lands of the public 

domain were sold by the Federal Gcvernment 

and thus constituted a source of national revenue. Upon passage of the Homestead Act, the 

national policy became the promotion of settlement and the development of the publi~ domain. 

The mineral land laws, enacted by Congress 

mainly between 1865 and 1875, had as their 

purpose the promotion of mineral development 

on the public domain. These Ia ws werE: based on 

local mining customs in force in the mining 

camps of that time. The act of May 10, 1872, 

provided for the exploration and purchase of 

mineral land on the public domain; it clearly 

was written to embrace metalliferous minerals, 

both lode and placer, for these were the mineral 

deposits known and sought at that tiJne. From 

time to time in later years Congress authorized 

the acquisition under the placer law.~ of nonmetalliferous minerals, which includerl petroleum, salines, and building stone. 

Public lands containing coal were ~.alt with 

by separate acts of Congress. The act of March 

3, 1873, provided for the purchase of coal land 

on the public domain at $10 an acre if the land 

1 Most of the history and many other parts of thif report were obtained from Smith and others (1913). 
was more than 15 miles from a railroad and at 

$20 an acre if the land was within 15 miles of a 

railroad. An individual was limited to 160 acres, 

an association of persons might acquire 320 

acres, and an association of four or more persons who had opened and improved a coal mine 

at a cost of more than $5,000 could purchase an 

area of 640 acres. 

Although it was not specifically stated, it is 

apparent from the language used in the many 

public land laws that the intent of Congress was 

to dispose of tracts in the public domain for 

whatever use they were best suited. For example, the Homestead Act stipulated " * * * nor 

shall any mineral lands be liable to entry and 

settlement." As early as 1785 the Continental 

Congress stipulated that each State entering the 

Union would be granted a part of ea~h township 

for schools. Ohio, the first State admitted to the 

Union (April 30, 1802), was allotted sec. 16 of 

each township. Later, other States were granted 

sees. 16 and 36, and a few States, sees. 2, 16, 32, 

and 36. The lands so granted were to be nonmineral, and wherever the stipulated sections 

were not available, the State was allowed to 

select other tracts in lieu of the allotted sections, but such tracts were to be nonmineral. 

Beginning on July 1, 1862, the railroad grants 

made by Congress to corporations to build railroads stipulated that the railroad corporations 

were to receive every alternate section of land 

in belts ranging in width from 20 to 40 miles on 

each side of the railroad; these sections were to 

include only nonmineral land, except for coal 

and iron land. 

Congress stipulated in the act of March 3, 

1879, which created the U.S. Geological Survey, 

that the Director of the Survey should classify 

the public lands. Except, however, for withdrawals of public lands for inclusion in reclamation projects and the segregation of reservoir 

sites, the classifications of land by the Geological Survey during the first 25 years of its existence were broadly based to identify features 

pertaining to land values for many purposes. 

President Roosevelt in 1906 directed the Secretary of the Interior to immediately withdraw 

from entry all valuable coal lands on the public 

domain. The Geological Survey, on the basis of 

its mass of scientific information on the public 

domain, prepared a list of public lands which 

had a reasonable probability of containing valu2 

able coal, and these lands were ]lromptly withdrawn from entry. Some additions were made to 

the withdrawals during the next. few years. It 

was not until June 25, 1910 (30 s~at. 847), however, that Congress passed the withdrawal act, 

although the earlier withdrawals had been in 

force since 1906. The Geological Survey, beginning in 1906 upon instructions from the Secretary of the Interior, was engaged in examining 

the withdrawn lands, and on the basis of these 

investigations the lands were classified; the 

lands found to be noncoal were restored to entry, those found to contain worl-able coal beds 

were appraised and priced at varying amounts 

per acre. 

Inasmuch as the lands withd~·awn included 

some lands that were valuable for farming, Congress later passed laws known as the "Separation Acts"-the act of March 3, 1909 (35 Stat. 

644) and the act of June 22, 1910 (36 Stat. 583) 

-which provided for a separation of the surface 

rights and the mineral rights and allowed the 

land claimant to obtain patent to his land by 

reserving to the United States tre coal deposits 

and the right to prospect and mine them. 

The Mineral Leasing Act of February 25, 

1920 ( 41 Stat. 407) provided for the leasing of 

coal and other mineral lands on the public domain and ended the practice of disposing of coal 

lands at appraised values. Under the provisions 

of the Mineral Leasing Act a permit to explore 

public land for coal may be obtained, or, if the 

land is known to contain workable coal, it is 

subject to competitive bidding for a lease. The 

operator is required to pay the Federal Government a stipulated royalty per tor of coal mined 

and an annual rental for the la.nd held under 

lease. 

During the four decades following the passage of the Leasing Act, the gee logic examination and classification of withdrawn lands continued at a reduced rate. In 196ll, however, an 

accelerated program of examination and classification of lands withdrawn for leasable minerals was begun. 

PURPOSE OF CLASSIFICATION 

In order to provide uniformity to executive 

action in the classification of co.-=~1 lands in the 

public domain, certain standardf for determining what lands are coal lands bave been prepared from time to time by the Geological Sur-
vey. It is the duty of the Geological Survey to 

determine what lands are underlain by coal 

within the limits set by these classification standards. Field examination to determine the presence of coal, thickness of beds, quality, attitude, 

depth, and other factors about the coal should 

precede classification. The field data should be 

assembled in such a form, including legible maps 

on adequate scale, as to facilitate the work of 

classification. Classification involves the consideration of all known geologic facts to determine 

what legal subdivisions of lands are coal lands 

under the provisions of the standards. 

The first guiding regulations for the classification and valuation of public coal lands, approved by the Secretary of the Interior, April 8, 

1907, prescribed a minimum workable thickness 

of coal of 2 feet and a maximum workable depth 

of 1,500 feet for coal land; all investigated lands 

that contained coal beds of less thickness or at 

greater depth were classified as noncoal. New 

regulations of April 15, 1908, retained the minimum thickness of 2 feet and prescribed a maximum depth of 3,000 feet for high-grade coal 

and 2,000 feet for certain thicknesses of lowgrade bituminous and subbituminous coal. On 

April 10, 1909, the Secretary of the Interior 

approved regulations which fixed a minimum 

thickness of 14 inches and retained the maximum depth of 3,000 feet; the heating value of 

unweathered coal and the tonnage of coal underlying the land were factors used in the classification. About 4 years later the following regulations were approved on February 20, 1913, by 

the Secretary of the Interior (Smith and others, 

1913, p. 96): 

1. Land shall be classified as coal land if it contains coal 

having: 

(a) A heat value of not less than 8,000 Btu on 

an air-dried, unwashed or washed, unweathered mine sample. 

(b) A thickness of or equivalent to 14 inches for 

coals having a heat value of 12,000 Btu or 

more, increasing 1 inch for a decrease 

from 12,000 to 11,000 Btu, 1 inch for a 

decrease from 11,000 to 10,500 Btu, 1 inch 

for each decrease of 250 Btu from 10,500 

to 10,000, and 1 inch for each decrease of 

100 Btu below 10,000. 

(c) A depth below the surface for a bed of coal 

6 feet or more thick of not more than 100 

feet for each 300 Btu or major fraction 

thereof, and for a bed of minimum thickness for that coal a depth of not more 

th~n 500 feet, and for beds of any thick3 

ness between the minimum and 6 feet a 

depth directly proportional to that thickness within these limits, providE'd that, if 

the coal lies below the depth limit but 

within a horizontal distance fron the surface not exceeding 10 times the depth limit, 

or if its horizontal distance fron the foot 

of a possible shaft (not deeper than the 

depth limit) plus 7.5 times the depth of 

such shaft does not exceed 10 times the 

depth limit, the land shall be elf' ssified as 

coal land; provided, further, that the depth 

limit shall be computed for eac" individual bed, except that where twc or more 

beds occur in such relations that they may 

be mined from the same ope .... ing, the 

depth limit may be determine~ on the 

group as a unit, being fixed at the center 

of weight of the group, no coal that is 

below the depth limit thus dete:-:-mined to 

be considered. 

2. Classification shall be made by quarter-qu'lrter sections or surveyed lots, except that for go"d reason 

classification may be made by 2%-acre tracts or 

multiples thereof described as minor subdivisions 

of quarter-quarter sections or rectangular lotted 

tracts. 

Detailed stipulations for appraisal ir dollars 

per acre of the coal lands were set forth in 

the regulations. They provided also that, after 

the valuation and the classification we1·e made, 

the lands should be restored to entry. After restoration, the coal lands could be acquired at the 

appraised prices and the noncoal land,so should 

revert to the status which they had prior to the 

withdrawal. 

A new set of standards for the clas8ification 

of coal lands was approved by the Acting Director of the Geological Survey on SeptE:mber 9, 

1959. The principal deviation from thr. regulations of February 20, 1913, was a chall ge from 

6 to 8 feet for the maximum thickness of a bed 

of coal. It is apparent, however, that a 6-footthick gently dipping coal bed can be clar,~ified to 

a· greater depth by using the 1913 re~pllations 

(Smith and others, fig. 3) rather than the 1959 

standards. The belt of land lying betwe~n these 

two depth limit lines would be classified as noncoal if the 8-foot maximum thickness is used but 

would be classified as coal land if th~ 6-foot 

maximum thickness is used. 

PRESENT STANDARDS 

The standards herein described for tre classification of coal lands provide for (1) a maximum depth of 6,000 feet, (2) a maximum depth 

of 1,000 feet for coals of minimum trickness, 
(3) use of Btu values for as-received coal, ( 4) 

a minimum heat value of 4,000 Btu, ( 5) a minimum thickness of 14 inches for all coals that 

have a heat value of 12,000 Btu or more, and (6) 

a maximum thickness of 6 feet for a coal bed 

that has a thickness of 6 feet or mo:--e (fig. 1). 

0 

500 

1000 

1500 

2000 

~ 2500 

j:.L1 

~ 

:z; 

H 3000 

::r: 

E--t 

p... 

~ 3500 

4000 

4500 

5000 

5500 

6000 

THICKNESS OF COAL BED, IN FEET 

1 2 3 4 5 

vl4 inches '~"\ ~ "' '\ ~ " t'-... 

1\.\ ~ f\. '\ I'\. ~ 

11\\ ~'\ "' '\ "' "' "'"' ~ 

~\ l\\' 1\. '\ 1\. "\ I": ~"'-

I~ l\\ 1\.\ 1\. '\ ~ "' .......... 

~~\ 1\\1\. '\ 1'\. f',_ !'--. 

~ 1\.\ """' 1\." "' 1'\. !'--. " ~ ~\ 1\' '\ "' '\ " ..... 

'~\\ [\'\ 

·"' 1\. 

\ "' " ' I\\' [\\ '\ '\ I~ 1\. "' '\~\ 1\. " "'i\. !'--. "'\. 

l\\ [\ '\ ' '\ '\ '\ "'\. 

\\ 1\\ [\\ '\'\ f\. f\. "\. 

\1\\ 1\.\ '\ i\. 1\. ' r'\. 

r\\ 1\.\ i\ ' "' "' "' 1'\. 

\r-..\ \ '\ '\f\. 1\. 1'\. 

\ ~1'\1\. ' \ 

\\I\ '\'\ 1'\ '\ '\ 

\1\\ \1\1\. 1\. 1'\ 

'\[\ 1\.' \\ \ \ 

\ 1\.' \ \ \ 1\. 1\. '\ 

\I\ \ \ \ \ 

1\ \\ 1'\ 1\ f\1\ \ 

'1\ 1\. \ \ \ 

"' \ r\ 1\ '"' I"' \ 

\I\ '\ I\ 

\I\ 1\ 1\ 1'\ \ ' 

1\ ' '\ 1\. 

\r\ 1\ 1\ 1\ \ \ 

' \ ' 1\ 1\ 1\ 1'\ 1\ \ 

\ \ \ \ 

1\ t\ 1\ \ 

\ \ \ \ 

1\ 1\ r\ 1\ \ 

\ \ ' 1\ 1\ 

\ \ \ 

' 1\ \ 

\ ' 

\ 

i'-.. 

~ 

'\ 

!"'-,.. '\ 

1'\ 

1--

1\ 

1'\ 

' 

\ 

~ 

1\ 

\ 

['.,. 

\ 

1\ 

\ 

1\ 

\ 

6 

000 I'. A 

~ 

!'\... 5 000 

"\. 6 000 

~ 7 000 

1\ 8 000 

\ 9 

'\ t--9 

['.,. 1-1 

~ 1 

000 

750 

0,000 

0,500 

1,000 

1,250 

'\ t-1 

1\ 

1-1 

\ 1 2,000 

\t-1 3,000 ~ 

\ t-1 4.000 

\ 

' 1 5,000 

FIGURE 1.-Diagram showing depth limits of coals of different Btu (as-received bas~s) 

and of different thicknesses less than 6 feet. 

4 
MAXIMUM DEPTH 

Smith and others (1913, p. 75) fixed a maximum depth of 5,000 feet for a coal that has an 

air-dried heat value of 15,000 Btu. They assumed for classification purposes "that a coal 6 

feet or more thick is workable to a depth of 100 

feet for each 300 Btu it contains." A maximum 

depth of 6,000 feet, which is 1,000 feet greater 

than the maximum prescribed in earlier regulations, is designated in the present standards 

because the advance in mi:r~ing technology since 

1913 appears to warrant the use of a greater 

maximum depth. Recent investigations of possible in situ exploitation of coal, if developed to a 

point where such operations are economic, may 

make a depth of 6,000 feet obsolete in the near 

future. 

MAXIMUM DEPTH FOR COALS OF 

MINIMUM THICKNESS 

A large increase in the mining of coal by stripping has taken place, particularly in recent 

years, since the regulations of 1913 were issued. 

The depth to which strip mining can be carried 

on economically has increased as machines and 

techniques have improved. Although the maximum depth of 500 feet prescribed in 1913 for 

coal of minimum thickness (Smith and others, 

1913, p. 75) is adequate for present operations, a 

depth of 1,000 feet may be more realistic for 

the future. Moreover, the change to 1,000 feet 

will make the classification of coal lands conform more closely to the data on coal reserves 

published from time to time by the U.S. Geological Survey (Averitt, 1969). The selection 

of 1,000 feet as the maximum depth for coals 

of minimum thickness is particularly important in the lignite fields of North Dakota. 

Brant (1953) stated that, except for some 

buttes in the southwestern part of the State 

where the lignite beds are at a depth between 

1,000 and 1,200 feet, all minable lignite is at a 

depth of less than 1,000 feet. Most of these 

buttes, moreover, are in an area that was classified as coal land many years ago. Inasmuch as 

the greatest part of the resources of lignite and 

subbituminous C coal is in nearly flat-lying surface formations in the Dakotas, eastern Montana, and northeastern Wyoming, it is almost 

entirely at a depth of less than 1,000 feet. 

Any land that contains a coal bed of the minimum thickness specified for its particular Btu 

value shall be classified as coal land for depths 

ranging between 0 and 1,000 feet. 

5 

USE OF AS-RECEIVED BTU V ALUF.~ 

The rules for classification are modified 

herein to the extent that heat values for asreceived coal shall be used instead of values for 

air-dried coal as in the past, because of the conditions in the laboratory where the analyses of 

coal are made. These conditions are described 

under the heading "Quality." 

In order to make this changeove~ without 

causing large changes in the depth limits for 

coals of various heat values, a comparison between air-dried and as-received Btu values of 

about 450 analyses of bituminous, s•lbbituminous: and lignite coals was made. It 'vas found 

that subbituminous B coal whose air-dried heat 

value is about 10,000 Btu has an as-rec~ived heat 

value of about 9,000 Btu. The comparison of coals 

of lower rank showed somewhat erratic differences, but comparison of coals whose heat value 

is more than 9,000 Btu showed the difference to 

diminish fairly uniformly between 9,000 and 

15,000 Btu. Accordingly, the spacing for the 

Btu scale in figure 1 between 9,000 and 15,000 

Btu is uniform at 333 feet per 1,00( Btu; between 9,000 and 4,000 the spacing is 400 feet per 

1,000 Btu. Although these two uniform sets of 

spacing do not agree precisely with the comparisons between the two scales that were made with 

large groups of analyses, they do not deviate 

from them by a great amount and they have the 

merits of simplicity and uniformity. 

MINIMUM HEAT VALUE 

The regulations for classification of coal land 

approved on February 20, 1913- (Emith and 

others, 1913) stipulated that any coal of proper 

thickness and depth tnat has an air-dried heat 

value of 8,000 Btu or more is considered workable and that land containing such coal is to be 

classified as coal land. Advances in c,1al beneficiation and in the designs of steam plants since 

1913," however, have made possible the commercial use of coal that has a much lower Btu value. 

Accordingly, it is considered advisabl~ to lower 

the minimum Btu requirement for classification purposes to 4,000 Btu on an 2s-received 

basis (fig. 1 ) . 

MINIMUM THICKNESS 

An investigation of the commercial mining of 

coal beds of various thicknesses was made in 

connection with the preparation of the regulations promulgated in 1913 (Smith and others, 

1913, p. 69). It was found that the ccst of min-
ing coals less than 6 feet thick increases as the 

thickness decreases, that coals yielding 10,000 

Btu were being worked "down to a thickness .of 

18 or 19 inches," that coals yielding 12,000 Btu 

were being worked "down to 14 or 15 inches," 

and that better coals were being mined from 

even thinner beds. In the light of these findings, a minimum thickness of 14 inches was fixed 

by Smith and others (1913) for all coals having 

12,000 Btu or more (air-dried basis), the minimum increasing 1 inch for a decrease from 

12,000 to 11,000 Btu, 1 inch for a decrease from 

11,000 to 10,500 Btu, 1 inch for each decrease of 

250 Btu from 10,500 to 10,000 Btu, and 1 inch 

for each decrease of 100 Btu below 10,000 Btu. 

The minimum thickness of 14 inches is retained in the present standards ; thus, the standards provide for a minimum thickness of 14 

inches for all coals having 12,000 Btu or more 

(as-received basis), the minimum increasing 1 

inch for each decrease of 750 Btu between 12,000 

and 9,000 Btu, and increasing 1 inch for each 

decrease of 250 Btu between 9,000 and 4,000 

Btu. 

In the classification of lands that contain beds 

of lignite in North Dakota, South Dakota, and 

eastern Montana, the practice of the Geological 

Survey has been to use a minimum thickness of 

2 feet 6 inches even though the heat value of the 

lignite varies rather widely throughout the region. Accordingly, in the present standards a 

minimum thickness of 2 feet 6 inches is fixed for 

a lignite that has an as-received heat value of 

6,000 Btu. Inasmuch as the minimum thickness 

of a coal that has a Btu value of 9,000 is fixed at 

1 foot 6 inches, which corresponds to the minimum thickness designated by Smith and others 

(1913, fig. 3) for the equivalent 10,000 Btu (airdried basis), the minimum thickness for coals 

whose heat value ranges between 9,000 and 6,000 

Btu (as-received basis) is increased 1 inch for 

each reduction of 250 Btu. Moreover, this same 

scale is extended to coals whose heat value 

ranges between 4,000 and 6,000 Btu (as-received 

basis). 

MAXIMUM THICKNESS 

The investigation conducted by Smith and 

others (1913, p. 74-75) of the cost of mining 

thick and thin beds of coal showed that there is 

little difference in the cost per ton for mining 

beds of coal 6 feet thick and those 10 feet thick 

6 

or thicker. Accordingly, a maximmn thickness 

of 6 feet was fixed in the regulations of 1913 and 

is retained in the present standards. Any bed 

whose thickness is more than 6 feet is treated as 

a 6-foot bed. 

THICKNESS OF COAL BEDS CONT AININC.. PARTINGS 

Coal beds that include partings have been 

treated uniformly by the Geological Survey for 

many years, as prescribed by Smith and others 

(1913, p. 70-71) after they had CC''1ducted an 

investigation. They concluded that any parting 

of bone or impure coal detracted fro'll the value 

of the coal an amount equal to the thickness of 

the parting. Two coal benches, each workable, 

are treated as parts of a single bed if the thickness of each bench exceeds the parting between 

them and the parting does not exce~~ 3¥2 feet. 

Two thick benches of coal are trerted as two 

beds if the thickness of the parting between 

'them exceeds the thickness of either one. A bed 

of coal 36 inches thick that includes a parting 6 

inches thick is regarded as being equivalent to a 

solid bed 24 inches thick. The thickness of the 

parting is deducted from the thirner of the 

adjacent benches above and below it. To calculate the total thickness of a coal bed, start with 

the best bench and add the thickness of the next 

bench above or below it after deducting from 

the coal bench a thickness equal to tl'~ thickness 

of the intermediate parting, anfl continue 

through the entire coal unit. If a parting is 

thicker than the thinner adjoining coal bench, 

that bench is considered as having nc value. This 

practice is best illustrated by a group o~ concrete examples. (See fig. 2.) 

TREATMENT OF COAL BEDS OF 

IRREGULAR THICKNESS 

As pointed out by Smith and others ( 1913, 

p. 72), the determination of the pro~able limits 

of workability of a coal bed that var~es in thickness from place to place is difficult. A few beds 

of coal may maintain a nearly uniform thickness over an area of several townships. In general, however, the thickness of mort coal beds 

varies widely and within short distances. The 

rules described by Smith and others (1913, p. 

72-73) and quoted below are followed in the 

present standards. 

The simplest problem is that in which two unequal 

measurements on a single bed, taken at different places, 

are available. All such beds are assumed to grade uniformly in thickness from the thicker measurement to or 
through the thinner measurement, and a liniit to the 

workable coal is thus fixed that, while it may or may not 

agree with the unknown facts, is the most probable limit. 

In general, a coal bed that can be traced continuously 

along the face of a cliff, as can many beds in the West, 

has usually been assumed to extend under the land back 

from the cliff at least one-half the length of the cliff outcrop, the lens or bed having the shape of a half-circle, 

the length of the cliff outcrop being the diameter of the 

circle. Obviously, if the outcrop runs along the cliff for 

many miles, the extension of the coal back from the outcrop may be modified by many other factors, such as 

limit of depth, or outcrop on the opposite side of the hill. 

If the bed outcrops along the cliff with irregular thickness, only a moderate extension of the bed behind the 

cliff is assumed, the estimate being based on the character of the irregularities shown by measurements made 

along the cliff, or by a general knowledge of the extent 

of the lenses of that particular coal or of coals of that 

a b a b a b a b a b 

·-··· 

1'2" ,. 6" 8" , .. II" ,.,, 2" ,.,, 4' 1'2" I' 2" 2'2" 1'6" 

I' 3" 2" 1'10" l'f¥' ,., .. 9" 1'8" 1'6" 

4" ,.,, II" 2" 2" 

1'2" 2" 1'6" 1'4" 

2'6" 2'6" 2'5" 2'5"- 2" 

1'4" 7" 

- 4" 9" 

3'4" 2'3" I'll" 2' 2' 

5'10' 5'7" 

a b a b a b a b a b 

10" I' I' 10" 10" 

9" I' 

6" 1'8" I'll" 7" 

1'10"1'8" ,. 6" 1'4" 2' 2' 3'9" 3'9" 

2" 2" - 2" 

1011 8" 

2'11" 2'11" 2" 1'8!' 1'6" 

1'10"1'10'' 1'4" 

3'6" 1'7" 3" - II" 9" 

1'3" 

2'1" 1'2" 4'0" 

3'10" 

5'9" 

FIGURE 2.-Diagram showing split or broken beds and 

their value for classification: a, measured thickness; 

b, equivalent thickness used in classification and valuation. From Smith and others (1913, fig. 2). 

7 

group, and depending on the general "habit'' of the bed, 

if known, and also taking into account all local features. 

If the exposed outcrop does not extend in a rtraight line 

but, as is more common, runs in and out of rf'.vines, careful note is made of the thickerung or thinning of the coal 

between one point of measurement and anot"'er in order 

to detect, if possible, any general tendency of the bed to 

thicken or thin in any direction, and all the~~ measurements and tendencies are taken into account in determining the probable extent of the lens in ary direction. 

Many beds studied are known to be of less than workable thickness in larger or smaller part, and any measurement showing a workable thickness on such a bed 

must usually be considered as a measure taken at the 

center of a small lens of workable coal. A discovery or 

measurement of a coal bed by a well, or by drilling, will 

have the same value as an isolated measure:-nent on the 

outcrop. The content and shape to be assirned to any 

lens must depend on conditions and are lar~~ely matters 

of judgment, and such assignments or determinations 

especially require a wide knowledge of coals and of 

the particular group of coals to which the bed under 

consideration may belong. 

Smith and others ( 1913) laid down explicit 

rule~ for treatment of special situations, such as 

groups of coal beds, coal beds in rugg1~d terrain, 

and coal beds adjacent to a shaft. Inasmuch as 

these rules are not changed in the new 

standards, they are quoted herein. 

TREATMENT OF A THIN COAL ASSOCIATED WITH 

A THICK COAL AND TREATMEl''T 

OF A GROUP OF COAL BEDS 

The following material is quoted from Smith 

and others (1913, p. 75-77). 

In general the limit of depth at which a coal bed may 

be profitably mined depends entirely upon the thickness 

of the bed and the quality of the coal. If, however, a 

thin but workable bed that lies below its ordinary minable depth is overlain by a thicker bed that lies within 

its own minable depth, the lower thin bed may, perhaps, be profitably worked by extending downward the 

shaft sunk to the higher, thicker coal, whereas the 

thinner bed could not have been profitably mined alone. 

A single shaft may also give access to all the coals of 

a group, and, though the cost of raising all the coal 

will be the same as if the coal were taken from a single 

bed, the cost of the shaft and the interest on that cost 

will be borne by the proceeds of a larger product. In 

some places the whole cost of a shaft may be borne by 

the coal of a thick bed that underlies sev,~ral thinner 

beds or that lies in the middle of a grour of thinn~r 

beds. It is clear that no two shafta that pene~rate groups 

of coal beds will present similiar condition:;". The coals 

of such groups will vary in number from tvro to twenty 

or more and may be included in an intel"1'•al of 50 to 

500 feet or more. Every case involving sue~ conditions 

must be conaidered on its merits, ar.d here again the 

Geological Survey has found lt necessary to establish a 

series of general precedents in order to make its treatment of such cases reasonably uniform. In some places 
it has been found possible to treat a group of coal beds 

as if they were all gathered at a "center of weight"-

that is, at a point determined by multiplying the computed thickness of each bed by its depth from the surface and dividing the sum of the products by the sum 

of the compensated thicknesses, the thicknesses of the 

various beds being reduced for partings in the manner 

already described. 

BURNED COAL 

Coal is commonly burned along the outcrop in 

most coal fields in the Western States. Because 

burning bakes and hardens rocks overlying coal, 

ledges of baked rock in many shades of red and 

orange are characteristic features of the western coal fields. Commonly, the baked rock is less 

than 50 feet thick, but in places it is several 

hundred feet thick. The extent of burned coal 

back from the outcrop is believed to be indicated 

by the width of the band of reddened rock and 

soil; its width may be only a few hundred feet 

where the slope above the coal is steep. and the 

cover is great, but its width may be one-quarter 

to one-half of a mile or more where the overburden is thin. The coal may be destroyed by 

burning where it lies near the tops of buttes and 

mesas. Characteristically, the band of burned 

rock is wide on narrow divides and narrow or 

absent in gulches. In fact, unburned coal commonly crops out in the troughs of gulches where 

they intersect the horizon of the coal bed. The 

field geologist should indicate the extent of the 

burned coal by appropriate symbols on his map. 

In classifying the land the edge of the coal is 

assumed to coincide with the boundary line for 

the burned areas. 

QUALITY 

The heat value of air-dried coal as expressed 

in British thermal units has long been used as a 

basis for determining the quality of coal for 

coal-land classification. As stated by Smith and 

others ( 1913, p. 68-69), coals that contain large 

percentages of mineral matter and moisture, 

which are the noncombustible constituents, show 

correspondingly lower heat values as expressed 

in Btu. On the other hand, coals that have small 

percentages of mineral matter and moisture 

contain large percentages of the combustible 

constituents in coal and therefore yield high 

heat values. It was these facts that caused the 

geologists who prepared the regulations of 1913 

to stipulate a minimum value in Btu for coal in 

the land to qualify it as coal land rather than to 

8 

set a maximum percentage of mineral matter 

and moisture in the coal as a deterr1ining factor. 

The rules for determining the rninimum heat 

value of any coal considered for classification of 

the land and the depth limits of C")als of different heat values and different thicknesses have 

been based in the past on Btu values on an airdried basis. Chemists of the U.S. Bureau ·of 

Mines who make the analyses have pointed out, 

however, that heat values on an as-received 

,basis more nearly represent the true condition 

of the coal in the ground; furthern1ore, they are 

more suitable for making compar~sons of coals 

of different rank. The Bureau of Mines discontinued reporting the analyses for the air-dried 

category many years ago because, according to 

R. F. Abernethy of the Bureau of Mines (written commun. to A. F. Bateman, Jr.~ U.S. Geological Survey, Jan. 3, 1961), 

The air-dry step in the preparation of a sample for the 

laboratory is an intermediate operatioP and can vary 

over quite a range. Samples of coal air-dried in winter 

may lose 5 to 8 percent more moisture than the same 

samples air-dried in the summer. This is due to the relative humidity of the air used in drying. The extremes in 

this laboratory run from ooF air heatec'l to 86°F in the 

winter time to air at 90°F and 90-100 percent relative 

humidity in the summer. The variation of the Btu on 

samples air dried under the above conditions would be 

so great they could not be compared. 

DIAGRAM (FIG. 1) SHOWING DEPTH 

LIMITS OF COALS 

The diagram, figure 1, was constructed to conform as closely as possible to th~ earlier diagrams and yet to incorporate th~ changeover 

from air-dried to as-received Btu values. First, 

the diagram, figure 3 of Smith and others ( 1913, 

p. 76), was modified to incorporate a maximum 

depth of 6,000 feet. As modified, the figure of 

10,000 Btu (air-dried basis) coincides with the 

4,000-foot depth line. A compilation of the airdried and as-received Btu values for about 450 

analyses of western coals showed that coals that 

have an average air-dried value of 10,000 Btu 

have an as-received value of 9,000 Btu. Accordingly, the 9,000-Btu figure was placed in figure 

1 at the 4,000-foot depth line. Tl'~ 15,000-Btu 

figure was placed at the 6,000-foot depth line, 

and the 2,000-foot interval betwe~n the 4,000-

and 6,000-foot depths was divided into six equal 

intercepts, each of which represents a heat increment of 1,000 Btu. Thus, each increment of 

1,000 Btu represents a depth change of 333 feet. 
The comparison of air-dried and as-received 

heat values for 450 analyses of western coals, 

referred to above, showed a more erratic distribution of values for coals that had heat values less than 10,000 Btu on an air-dried basis 

and 9,000 Btu on an as-received basis, presumably because of the higher moisture content of 

these coals, which in turn gave less uniform 

values when the coals were air dried in the laboratory. If the as-received values were to be 

placed on the diagram scale at the precise positions that correspond with the air-dried Btu values determined by the study of 450 analyses, the 

intercepts between 6,000 and 9,000 Btu would 

range between 180 and 340 feet on the depth 

scale and the intercepts for Btu values less than 

6,000 Btu would have a much greater interval. 

It seems more sensible to apply a uniform scale 

to the Btu values for a diagram such as figure 1 

rather than to plot irregular intervals based on 

comparisons of available analyses. The deviation from the actual value will not be large for 

any group of coals. Accordingly, as drawn, an 

increment of 1,000 Btu between 4,000 and 9,000 

Btu represents a depth change of 400 feet. 

As explained under the heading "Minimum 

thickness," the diagonal line that extends upward from the 6,000 Btu point terminates at 2 

feet 6 inches because this thickness corresponds 

with the commonly used thickness cutoff for 

lignite that has low heat values. Thus, the difference in minimum thickness for coals whose 

heat values range between 6,000 and 9,000 Btu 

is 4 inches for each 1,000 Btu, or 1 inch for each 

250 Btu. This scale is extended through the 

interval between 6,000 and 4,000 Btu. 

The user of figure 1 should ( 1) obtain the asreceived Btu value from chemical analyses of 

unweathered mine samples of the coal, (2) plot 

this Btu figure on the scale at the right margin 

of figure 1, (3) draw a diagonal line upward to 

the 1,000-foot depth line to determine the minimum thickness for the coal, and ( 4) if the coal 

is thicker than the minimum thickness, read the 

d~pth limit at the intersection of the diagonal 

line and the vertical thickness line. For example, 

an 8,000-Btu coal has a minimum thickness of 1 

foot 10 inches. All land that contains this coal 

bed at depths between 0 and 1,000 feet should be 

classified as coal land. If the coal bed is 4 feet 

thick, its depth limit is 2,360 feet instead of 1,000 

feet; if its thickness is 6 feet or more, its depth 

9 

limit is 3,600 feet. Another example is a coal bed 

that has an as-received Btu value of 5,( '10; if the 

bed is 2 feet 10 inches thick and at d~pths between 1 and 1,000 feet, the land containing it 

should be classified as coal land. On the other 

hand, if the coal bed is less than 2 feet 10 inch,es 

thick, the land should be classified as noncoal. 

AREAL BASIS FOR CLASSIFICATION 

Classification shall be made by quarterquarter section ( 40 acres), surveyed tract, or 

surveyed lot. 

SUMMARY OF THE STANDARDS FOR THE 

CLASSIFICATION OF PUBLIC COAl. LANDS 

1. Land shall be classified as coal land if it contains coal having: 

(a) A heat value of not less tl~an 4,000 

Btu (as-received basis) for an 

unwashed or washed, unweathered mine sample. 

(b) A thickness of 14 inches for coals 

having a heat value of 12,000 Btu 

or more (as-received basis), increasing 1 inch for each decrease 

of 750 Btu between 12,000 and 

9,000 Btu, and 1 inch for each 

decrease of 250 Btu between 9,000 

and 4,000 Btu. Any coal t'~d whose 

thickness is more than 6 feet is 

treated as a 6-foot bed. I11 calculating the thickness of a coal bed that 

contains partings of shale, bone, or 

impure coal, the thickne<:1,'J of the 

thinner bench of coal directly 

above or below the part~ng is reduced by the thickness of the parting; thus, the total thickness of 

the coal bed (including partings) 

is reduced by twice the tc tal thickness of the partings. 

(c) A depth of not more than (\ 000 feet 

for a bed of coal 6 or r"lore feet 

thick having an as-received heat 

value of 15,000 Btu. The depth decreases 333 feet for each decrease 

of 1,000 Btu between H\000 and 

9,000 Btu and decreases 400 feet 

for each decrease of 1,00~ Btu between 9,000 and 4,000 B+u. For a 

bed of minimum thick:1ess, the 

depth may not be more than 

1,000 feet. For beds of any thick-
ness between the minimum and 6 

feet, the depth is graduated between 1,000 feet and the maximum depth for a 6-foot bed. 

Moreover, the depth limit shall be 

computed for each individual bed 

except that, where two or more 

beds occur in such relations that 

they may be mined from the same 

opening, the depth limit may be 

determined on the group as a unit 

and is fixed at the center of 

weight of the group; no coal below 

10 

the depth limit thus determined 

is to be considered. 

2. Classification shall be made by quarterquarter section, surveyed tract, or surveyed 

lot. 

REFERENCES CITED 

Averitt, Paul, 1969, Coal resources of the United States, 

January 1, 1967: U.S. Geol. Survey Bull. 1275, 

116 p. 

Brant, R. A., 1953, Lignite resources of North Dakota: 

U.S. Geol. Survey Circ. 226, 78 p. 

Smith, G. 0., and others, 1913, The classification of the 

public lands: U.S. Geol. Survey Bull. 537, 197 p. 

'tru.s. GOVERNMENT PRINTING OFFICE: 1970 o-402-662