330 DESCRIPTIONS OF MINERALS. 
The following are average results, from many analyses : 


Vol. |Fixed 
Nos.| Sp. gr. | com- | Car- | Ash. Analysis. 
bust. | bon. 


7 159-161 3°92 | 89°77 | 6°31 | Johnson. 
16 |1°39-1°60 | 5°70 | 88°23 | 607} Geol. Survey. 
11 |1°33-1°45 | 9°98 | 82°86 | 7:16} Geol. Survey. 
6 |1°30-1°41 | 16°85 | 72°95 | 10°20 | Johnson. 
10°47 | § Johnson and 
Pennsylvania anthracites... 
2 | Pennsylvania semi-anthra- 
CECE a deca oes ee 
3 | Pennnsylvania semi- bitumi- 
MOUSRE Rano ee eter { | 
ae eet 
4 | Maryland semi-bituminous... 9 ‘1:80-1:43 | 15 50 | 74 03 '7 Geol. Survey. 
5 | Pennsylvania bituminous....) 10|......... | 28°35 | 65°18 | 6°47 | Johnson. 
6 | Virginia bituminous..... ... 11 |1-29-1°45 | 29°88 | 59°06} 11:06 | Johnson. 
7 | Ohio bituminous............-. 142 |1:24-1:47 | 35-24 | 60°26 | 450] Wormley. 
8 | Indiana bituminous.......... 126) |119=1-4 1" | 43520 (53°47 | oa eos 
9 | Hlinois bituminous........... 59 |1-21-1°35 | 31°90 | 62°44) 566) Blaney. 
107) owas bitummMoush. see eee HON Meenas Secon tae 43°02} 682) Emery. 








The ordinary impurities of coal, making up its ash, are Sitica, a 
little potash and soda, and sometimes alumina, with often oxide of 
iron, derived usually from sulphide of iron ; besides, in the less pure 
kinds, more or less clay or shale. The amount of ash does not ordina- 
rily exceed 6 per cent., but it is sometimes 80 per cent.; and rarely it 
is less than 2 per cent. When not over 8 or 4 per cent. the whole may 
have come from the plants which contributed the most of the material 
of the coal, since the Lycopods have much alumina in the ash, and 
the Equiseta much silica. 
There is present in most coal traces of sulphide of iron (pyrite’, suf- 
ficient to give sulphur fumes to the gases from the burning coal, and 
sometimes enough to make the coal valueless in metallurgical opera- 
tions. Some thin layers are occasionally full of concretionary pyrite. 
The sulphur was derived from the plants or from animal life in the 
waters. Sulphur also occurs, in some coal beds, as a constituent of a 
resinous substance ; and Wormley has shown that part of the sulphur 
in the Ohio coals is in some analogous state, there being not iron 
enough present to take the whole into combination. 
The average amount of ash in eighty-cight coals from the southern 
half of Ohio, according to Wormley, is 4°718 per cent.; in sixty-six 
coals from the northern haif, 5°120; in all, from both regions, 4891 ; 
or, omitting ten, having more than ten per cent. of ash, the average is 
4°28. In eleven Ohio cannels, the average amount of ash was 12°827. 
The moisture in the Ohio coals, according to the analyses of Wormley, 
varies from 1:10 to 9 10 per cent. of the coal. 
Mineral coal occurs in extensive beds or layers, interstratified with 
different rock strata. The associate rocks are usually clay shales (or 
slaty beds) and sandstones ; and the sandstones are occasionally coarse 
grit rocks or conglomerates. There are sometimes also beds of lime- 
stone alternating with the other deposits. ' 
Coal beds vary in thickness from a fraction of an inch to 40 feet. 
The thickness of a bed may increase or diminish much in the course 
of a few miles, or the coal may become too shaly to work. 
