458 ELEMENTARY GEOLOGY 
It is such variations as these in the rock record, 
that have convinced the geologist of the constant 
variety in the history of the past. Now there is dry 
land, and then very soon, its place is occupied by 
the ocean; and again, perhaps after the lapse of 
some time, there is another change back to the con- 
dition of dry land. The sea is now shallow and 
again deep; or now under the influence of the land, 
when the deposits are either muddy, sandy, or pebbly, 
while later the water has become clear, and so free 
from land sediment that beds of limestone can accumu- 
late. Evidence of constant change, operating slowly 
and even imperceptibly, is ever seen by the student 
of the earth’s crust. No doubt these same changes 
are even now in progress; and could we watch them 
for many centuries, we would find the same variations 
in the conditions of the present, as are so plainly 
recorded among the rock leaves of the earth’s history. 
Carboniferous Geography. — The great Paleozoic sea, 
in which much sediment derived from the neighboring 
mountains had been accumulating, was destroyed dur- 
ing the Carboniferous period, and the land east of the 
Mississippi River was permanently changed to dry land. 
This elevation accompanied the growth of the Appala- 
chian Mountains. Throughout all the immense ages of 
the Paleozoic, the bottom of this inland sea was slowly 
sinking ; and then, towards the close of the Carbonifer- 
