4 PROCEEDINGS OF THE DETROIT MEETING. 
not been observed. Carbonaceous materials are generally present as fragments of 
plants, as vegetal ooze in greater or less proportion to the other constituents, and 
as distinct coal beds. Carbonate of iron is frequently an integral constituent of 
the rocks. 
In color they are, when fresh, generally bluish gray, shading to brownish black. 
They weather to buff tints, which are usually dull. The coarser and more massive 
varieties form beds 20 to 100 feet thick, in which bedding planes are not distin- 
guishable. The finer deposits are thinly laminated and carry abundant leaf im- 
pressions, which occasionally interlap with one another so as to form a mass of 
leaf fragments. 
The weathered forms assumed by these rocks rarely present sharp profiles. The 
more massive beds develop rounded bosses by spheroidal disintegration due to 
oxidation of the iron carbonate. The thinly bedded strata break down readily, 
either as a clay mud or in thin scaly fragments, or in angular bits which are ex- 
ternally indurated by a cement of iron oxide. 
In many of the sandstones silvery white mica has developed as a secondary min- 
eral, but they exhibit no other indication of metamorphism. The coals, on the 
contrary, being chemically more sensitive. have undergone metamorphism to a 
greater or less extent through loss of combined water and concentration of fixed 
carbon. They vary, therefore, from lignites, whose representative analyses have 
the range— 
Per cent. 
IMIS) ERLE pase er enceanaadbcpacton cna nbeasabenoosaceidadaccteraonoscdseeracodsupospaccood 8 to 12 
Wolken mille, any ebuoyer be oO ss ocqeacsoocisocsdboce-coceecoacoonoceneenopecdburEuaigncscs: B18) (Wo) 2's) 
Hie dicanbonescncarecccteaueccsesceser rere cerasoeteeseiece rec eceate arse ace 30 to 45 
to bituminous lignites or steam coals, in which the moisture is reduced to 5 per cent 
or less, and the fixed carbon ranges from 40 to 50 per cent, or to bituminous coking 
coals, which are fairly represented by the figures: 
Per cent. 
MOISbures.gecsreeete eh einen ubeinensaniesecccs tne beesemteres eee stenee obese aioe} 
WON IIIS Tiny Cb XOXOE TA NONI SS cd aceeo/ coosexconecececonondobunrinoosctedonanoas0sqs00 25 to 35 
Hix ed. (Carbon encertacncscuscnacie cocee moore vaccons Antaneees seers cnerpe eee eeceer 50 to 60 
The variations from lignite to bituminous coking coal are of regional extent— 
that is tosay, where lignites are found they may be expected to maintain a uniform 
composition over a relatively wide area, and bituminous varieties are equally con- 
stant in their character within the fields in which they occur. There are, however, 
occurrences of more condensed coals, ranging into anthracite, which are, so far as 
is definitely known, of local distribution only. , 
The cause of variation in quality among these coals may be sought in pressure 
and movement which they have suffered. The lignites retain the compact struct- 
ure originally assumed by the peaty deposit under the load of overlying strata. 
Their beds have been tilted, but internally not much disturbed. They have there- 
ore undergone comparatively moderate chemical change. The Green River steam 
coals have assumed a more or less cubical structure, due to shearing under press- 
ures which caused movement within the vein. The resulting chemical effect was 
to expel 5 to 8 per cent of water. Beyond the area of this mechanical influence 
the coal changes into lignite by transition within a single bed. The coking coals 
of the Wilkeson field and those of the extreme eastern portion of the Green River 
field have been rolled out between their walls and crushed. Their softness and 
their concentrated condition have resulted from this mechanical disturbance. The 
x 
