274         CONTRIBUTIONS    TO   ECONOMIC   GEOLOGY,   1907,   PART   II. 
Values  of  coal  in  the  Rock  Springs  coal  field,  Wyoming. 
Coal  group. 
Rock  Springs. 
Almond 
Black  Buttes. 
Black  Rock.. 
Number 
of 
samples. 
Range. 
[Maximum 
36  v  Minimum 
(Average 
Maximum 
12   { Minimum 
(Average 
'[Maximum 
5  k  Minimum 
i  I  Average 
i  Maximum 
6  < Minimum. 
I  (Average 
Ash. 
Sulphur. 
Carbon. 
15.80 
1.52 
71.85 
1.51 
.36 
44.75 
4.60 
.96 
62.  91 
11.74 
.96 
65.  32 
4.50 
.32 
52.94 
7.11 
.56 
57.94 
4.84 
.64 
62.59 
4.36 
.52 
62.59 
4.52 
.56 
62.  55 
17.41 
1.48 
63.55 
4.94 
.30 
43.16 
8.44 
1.00 
57.46 
Fixed 
carbon. 
55. 17 
31.58 
48.59 
59.98 
41.97 
47.24 
55.  96 
47.94 
51.77 
52.  78 
28.17 
42.83 
On  computing  the  carbon-hydrogen  ratio  a  for  the  analyses  given  in 
the  table  on  pages  270-272,  it  will  be  found  that  there  is  a  great  irregu- 
larity in  the  order  of  the  heat  values  of  the  coal  as  given  by  this  ratio 
and  the  actual  determined  British  thermal  unit  values,  the  difference 
on  many  samples  being  from  1,500  to  2,500  British  thermal  units. 
The  carbon-hydrogen  ratio  apparently  bears  no  regular  relation  to  the 
heat  values  of  the  coal.  On  the  other  hand,  a  comparative  study  of 
ultimate  analyses  shows  that  in  nearly  all  kinds  of  coals  the  ash  and 
oxygen  are  of  nearly  equal  importance  as  impurities  from  the  stand- 
point of  heat  efficiencies.  On  computing  the  carbon-oxygen  ratios 
and  arranging  the  values  according  to  the  ratio  C-^(0  +  ash),6  it  is 
found  that  these  values  have  approximately  the  same  order  as  the 
efficiencies  determined  calorimetrically.  Although  the  order  differs 
slightly  from  the  British  thermal  unit  values,  the  difference  is  small, 
being  for  the  greater  number  of  samples  less  than  100  and  seldom  ex- 
ceeding 300  British  thermal  units.  The  only  notable  exception  is 
No.  6042  of  the  table,  where  the  ratio  C^-(0  +  ash)  places  the  coal 
approximately  900  British  thermal  units  lower  than  the  calorimet- 
rically determined  value.  Many  of  these  coals  are  greatly  weathered, 
and  it  is  remarkable  that  these  ratios  should  correspond  so  closely 
with  the  actual  determined  British  thermal  unit  values.  It  appears 
that  the  ratio  C  ~  (O  +  ash)  furnishes  a  fairly  satisfactory  basis  for 
classifying  or  grouping  the  coals  according  to  their  heat  efficiencies. 
COMPARATIVE  VALUES. 
The  lower  Mesaverde  or  Rock  Springs  coals  occur  in  the  same 
geologic  formation  as  the  high-grade  coals  of  the  Yampa,  Danforth 
Hills,  and  Grand  Hogback  fields  of  Colorado,  and  the  Book  Cliffs  field 
of  Utah,  and  compare  favorably  with  them.  These  coals  are  not  so 
good  as  the  coals  of  Benton  age  in  Uinta  County,  which  show  a 
tendency  to  coke  and  have  a  higher  heating  efficiency.     The  Rock 
o Campbell,  M.  R.,  The  classification  of  coals:  Trans.  Am.  Inst.  Min.  Eng.,  vol.  36,  1905,  p.  324;  Prof. 
Paper  U.  S.  Geol.  Survey  No.  48,  1896,  pp.  156-173. 
6  White,  David,  Oxygen  values  and  coal  alteration;  Science,  new  ser.  vol.  27,  1908,  p.  537. 
