108 
GEOLOGY OF NORTH CAROLINA. 
continent, having been thus modified by the very circumstances of dis- 
turbance and contortion to which they were subjected in the process of 
upheaval. 
The mode of formation of the different kinds of stratified rocks can 
only be understood by observing the different conditions which now de- 
termine the deposition of one or another of these kinds of materials in 
moving water. 
Thus it is seen that the materials brought down by rivers and floods 
from land surfaces, or abraded from sea shores by the action of waves, 
are deposited in a certain order of arrangement ; the coarser gravels and 
sand drop at the first slackening of the current, as the waters debouch 
into the bays, estuaries, &c., and along the shores, and the finer materials 
are carried further out and into deeper water; and it is found that at 
depths greater than a few hundred feet, and in all open 6eas, but little 
less else is deposited besides calcareous sediments, consisting of the hard 
parts of minute organisms which inhabit the still depths and secrete their 
solid particles from the carbonate of lime held in solution by the sea wa- 
ter and continually supplied by the rivers, — accumulations which are 
afterwards changed into chalk, limestone, marble, &c. 
By widely extended and long continued observations in many parts of 
the world, it is ascertained that the sea level is everywhere aud continu- 
ally changing with respect to its shores, now advancing, now receding, 
and that these oscillations are due to the alternate rising and sinking of 
the land masses. And as the sea encroaches upon the land, the coarser 
sediments that were dropped along the shores, are gradually carried back 
into deeper water and overlaid by finer and finer materials, first sand, 
then clay, and at length, by the still finer calcareous mud. And hence 
the usual order of arrangement among stratified rocks ; first the coarser 
conglomerates and sandstones, followed by clay slates and shales, and 
finally by limestones. Hence also the former deposits contain, for the 
most part only the remains of land plants and animals, often beds of drift- 
wood, while the fossils found in the shales and limestones are entirely of 
marine origin, the remains of shell fish, corals, fishes, &c. 
It is obvious that in the progress of the changes above considered, as 
the sea advances upon the land, its successive deposits will be laid down 
over all the irregularities, the excavated river channels, the upturned and 
broken rocks and the hills and valleys of the land surface ; and so, when 
in the reverse process, the ocean bed gradually rises to the air and be- 
comes dry land again, it will be re-peopled by animals and plants, and 
channelled anew by rivers, and its surface eroded by rains and frosts into 
