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AGENCIES IN THE FORMATION OF ROCKS. 143 
so great as to form large tracts of land, as at the protruding mouths 
of the Ganges, Nile, or Mississippi. Even in historic times, the 
land thus gained is of great extent. For example, at the mouth of 
the Po, a minor stream, the town Adria, which gave its name to the. 
Adriatic Gulf from its extensive commerce in Roman times, is now nine: 
miles from the sea! The mass of matter held in solution or borne along 
by the running water, sinks to the bottom when it reaches the sea, in a 
certain order. First the heavier masses are deposited, such as boulders and 
gravel ; then, the sand ; and last, the mud. Mingled with these are various 
animal and vegetable remains that have been washed into the stream. 
Thus, every river-mouth presents an ever-growing series of beds of varying 
thickness and material, superposed the one on the other, and enclosing 
various remains of animal and vegetable life. These strata would, in 
the above order, be converted, by pressure, into conglomerate, sandstone, 
slate, shale, and coal. Thus, again, we have found a beautiful and perfect 
explanation of the Stratified rocks as they are presented everywhere, by 
_ which their composition, stratification, and contents are fully accounted 
for. Stratified rocks, therefore, obtain the various names of sedimentary, 
because formed of the sediment of rivers ; and aqueous, because deposited 
under water. 
Organic Agents—But animal and vegetable life is also busy in the 
formation of rocks. Away in the warmer seas of the Pacific, lives the 
coral insect or zoophyte, the skeletons of which compose the remarkable 
coral reefs that form the chief part of the numerous isles that stud that 
_ greatest of seas. These reefs extend thousands of miles, in broad barriers, 
over which the wild waves dash, or in detached groups that gradually 
gather round them material and form new islands.’ In the rocks, we also 
find the remains of like corals, standing where they grew, or drifted away, 
and appearing as extensive formations of limestone. 
Again, the bottom of the sea is covered with accumulations of minute 
shell-fish, of great depth and over extensive areas, as is proved every 
day by soundings with the lead. Now, the old rocks exhibit strata of 
identical composition with these microscopic shells; some limestones and 
chalks, for example, being composed of millions to the square inch of 
perfect bivalve shells, Again, the sea-bottom contains beds of shell-fish, 
of different kinds, and of great extent and thickness. Should these die, 
and then be subjected to sufficient pressure, they would form a rock, 
exactly like the shell limestones so common in our rock-formations, and 
so valuable in agriculture and building. 
Then we have the remains of ancient forests in our great mosses ; and 
Tuxuriant growths of swampy plants and impenetrable jungle in the 
mud islands of the deltas of our great rivers in the tropics. These, 
1 From Latin aqua, water. 
