138 GEOLOGICAL SURVEY OF ALABAMA. 
for instance, alternations of limestone with sandstones and 
conglomerates, while, in point of fact, the sediments which 
make the rocks of these older formations are many thousand 
feet in thickness and consist of sandstones, conglomerates, 
shales, and limestones in many alternations. 
All this is clear demonstration that the floor of the sea 
did not remain stationary during this period, but subsided, 
—at least to the extent of the thickness of the sediments 
accumulated upon it,—not steadily and continuously, but 
with many pauses of downward movement, alternating even 
with movements in the opposite direction, which went so 
far at times as to bring parts of the sea bottom above the 
water, and to afford the requisite conditions for the accumu- 
lation of those immense beds of vegetable matter that con- 
stitute the seams of coal. 
In the manner above sketched, there were accumulated 
upon the floor of the interior sea, and in the marshes and 
peat bogs of the land, and in the estuaries of the rivers, 
during a period of whose duration we have no means of 
making a definite estimate, beds of gravel, sand, mud and 
limestone, and coal beds, of varying thickness according to 
position ; from 40,000 feet near the margin of the sea where 
the greater part of the land waste was deposited, to 4,000 
feet further out to sea where the materials deposited were 
mainly calcareous and siliceous. These beds contain the 
remains of the animals and plants that flourished upon the 
land or in the waters of the ocean during the period of their 
accumulation, and when consolidated and elevated above 
sea level they constitute the rocks of the various geological 
formations. These rocks and their contained organic re- 
mains, have been objects of study and investigation among 
geologists for many years, and as one of the results of these 
investigations, they have been classed together into a num- 
ber of great groups having certain common characteristics 
of mineral composition and fossils. The names of these great 
geological groups or formations beginning at the lowest and 
proceeding upwards, are Cambrian, Silurian, Devonian, and 
Carboniferous. The maximum thickness of the rocks of 
these formations, as they are displayed in Alabama, may be 
approximately given as follows: Cambrian 10,000 feet; 
