460 ELEMENTARY GEOLOGY 
ous, the reverse motion occurred, and it rose above the 
ocean. 
Throughout the earlier part of the Carboniferous, 
conglomerates, sandstones, and shales continued to be 
deposited near the eastern shores in Pennsylvania, and 
the states south of this; but to the westward, in the 
great Mississippi valley, the sediments were of finer 
texture. About. the middle of the period, the sea shal- 
lowed in various places and soon became transformed 
to marshy lands. The ancient shore-line, now occupied 
by the Appalachians, was then fringed by extensive 
marshes ; and similar tracts of swampy land existed in 
Illinois, lowa, Texas, and other parts of the interior. 
On these swampy districts vegetation grew in great 
abundance. There were veritable jungles with trees of 
weird form,— gigantic ferns, cicads, lepidodendrons, 
etc. The climate of the period was equable and the 
vegetation had a tropical aspect. The atmosphere was 
humid because of the great extent of water and swamp, 
and some believe that it contained much more carbonic 
acid gas than is now present. It 1s thought by many 
that the coal-forming vegetation withdrew this gas 
from the air and accumulated it into beds, so that the 
atmosphere after the coal period, or Carboniferous, be- 
came suitable for the existence of the higher forms of 
land life. It must be said, however, that this is merely 
theory, against which facts can be raised. 
ee ee 
