The time required must have been very great to
accumulate vegetable matter, principally composed of
fine spore dust, to a depth sufficient under great compres-
sion to give even a foot of solid coal. Dr. Dawson, who
has devoted great attention to the coal-fields of America,
says : " We may safely assert that every foot of thick-
ness of pure bituminous coal implies the quiet growth
and fall of at least fifty generations of Sigillaria, and
therefore an undisturbed condition of forest growth,
enduring through many centuries." But this is only
the first step in the measure of the time required for the
formation of the coal measures. Each seam of coal is,,
as we have seen, covered by a bed of sand or shale, i.e.,
of water-borne materials. How can this be accounted
for ? Evidently in one way only — that the land surface
in which the forest grew subsided gradually until it
became first a marsh, and then a lagoon or shallow estu-
ary, which silted up by degrees with deposits of sand or
mud, and, finally, was upraised until its surface became
dry land, in which a second forest grew, whose debris
formed a second coal seam. And so on, over and
over ao'ain, until the Avhole series of coal measures had
been accumulated, when this alternation of slight sub-
mergences and slight rises came to an end, and some
more decided movement of the earth's surface in the
locality brought on a different state of things. This
is in fact exactly what we see taking place on a smaller



32 MODERN SCIENCE AND MODERN THOUGHT.
