6 BULLETIN 102, PART 1 , UNITED STATES NATIONAL MUSEUM. 
ness of layers. Ultimately, however, the proceeding enters into an- 
other stage in the form of a reversal to a rising instead of a subsid- 
ing movement. This in due season means the final reappearance 
of the area as dry land with the one-time swamp accumulation of 
organic matter buried deep down in the earth. The pressure of the 
vast weight of materials above, coupled with the various other geo- 
logic agencies needless to recount here, will have compressed and 
condensed the peaty aggregate to coal and welded the sediments 
into beds of shale, sandstone, and limestone, as exemplified in plate 5. 
With the emergence of the region as dry land, the process of coal 
evolution enters into its closing stages. Erosion or the gradual wear- 
ing down of all land areas provides the reverse aspect of accretion 
or the building up of sea-bottom areas; and the rate of erosion is 
commensurate with the abruptness of elevation. It is not to be ex- 
pected that the dynamic forces behind the upward movement would 
necessarily produce an even uplift; rather it is to be expected that 
zones of relative weakness would develop and that the uplift would 
be uneven. Erosion then would proceed unevenly over the area; 
and in the course of geologic time might here and there succeed in 
wearing completely through the overlying rock mantle, even cutting 
through the coal and far into the underlying rock. The result would 
be the outcrop of a coal bed, and from that point forward the ma- 
terial history of coal, till then involving time in periods defying any 
attempt at reference to human standards of measurement, is ready 
to merge into the history of man. 
From the foregoing brief sketch of the history of coal evolution, 
it will appear by way of introduction to what follows that coal is 
a development from organic accumulations, effected presumably in 
vast marine swamp areas, and the various classes of coal represent 
so many different stages in the transition from vegetal to stony con- 
dition of occurrence; that the transition is induced primarily by 
the physical element of pressure with its accompaniment of heat; 
and that accordingly the anthracite stage of the evolution does not 
necessarily imply a formation of greater age, but rather marks a 
more extreme phase of metamorphism whether from duration or 
from intensity. The natural tendency from external pressure is 
toward condensation, and that is precisely the nature of the evolu- 
tionary development wrought by the dynamic forces of pressure and 
heat in coal. Physically it takes the direction of a progressive con- 
densation toward the stony extreme of anthracite; and chemically, 
as may be observed in the chart, there is a corresponding tendency 
in the progressive elimination of the lighter, gasifiable, or volatile 
ingredients. These are the conditions native to the occurrence of 
coal which furnish the groundwork for the structure of industrial 
usage. 
