650 THE JOURNAL OF GEOLOGY. 
immediately underlying strata. In other instances, as for exam- 
ple the ‘‘second seam” in the Eureka mine at Des Moines, the 
usual rule holds true. 
If we conceive the coal seam at the time of its formation, we 
would have essentially what may be seen in any modern peat 
swamp. The loose vegetal matter lies under the water com- 
pletely covering the irregularities of the bottom and rising toa 
practically uniform level a short distance below the surface of 
the water. In the process of change by which the vegetal mat- 
ter is transformed to coal it loses a considerable percentage of 
gas and water so that the relative percentage of carbon increases 
in regular order from wood to anthracite. 
Bischof in studying these changes calculated the total loss 
which takes place in the change of wood into coal. He assumed 
that the change had taken place by three processes: 
I. By the separation of carbonic acid and _ carburetted 
hydrogen. 
By the separation of carbonic acid from the elements of 
i) 
wood, and by oxidation of hydrogen by external oxygen. 
3. By the separation of carbonic acid and water from among 
the elements of wood. 
Under these three assumptions the wood lost as follows: (1) 
7s) [se Cems (A) Gidea oer Cet (2) AS.8 joer cent, 
It is readily seen that with so great a loss of material there 
must be a corresponding decrease in volume. As the gases 
escaped, a process yet in operation in the deeper mines of 
other regions, the strata settled. The amount of this settling 
may be arrived at as follows. 
In the coal basin figured above it will be seen that at one 
point the floor of the coal bed is sixty feet above the lower 
portion adjoining. At the time of the deposition of the vegetal 
matter both these points were covered to a considerable depth, 
the loosely compacted material rising probably to an even level. 
It is evident that at that time there was sixty feet of material 
less over the high than the low point. The whole mass became 
so deeply buried that the pressure of the overlying beds may be 
