PRELIMINARY REMARKS. 
25 
mass, and fall into the ocean. These are icebergs. They then 
drift away, and are carried by currents out to sea, and often 
reach, in the northern hemisphere, the latitude of the Azores. 
On their march, they melt and distribute their burthens of earth 
and rock over the ocean’s bottom. It is maintained by eminent 
geologists, that glaciers were formerly far more extensive than 
now, and hence there was a period which deserved the name of 
glacial period. 
§ 21. The general tendency of the operation of water, as 
described in the foregoing paragraphs, is to form accumulations 
on the lines and planes of flow’, or a little outside of them; 
which, beginning upon the mountain tops, terminate in the 
broad ocean rivers. The coarsest matter is left near the sum¬ 
mits, and the finest is carried to the bottoms of the ocean rivers. 
Another belt of coarse materials is along the ocean shore and 
margins of lakes. These materials are mostly hard and sili¬ 
ceous, and resist change a long time; but they become pebbles 
and sand by attrition. These form the conglomerates A Farther 
out from land there is a mixture of fine silex and clay; and 
still farther, in the flow of ocean rivers and currents we find 
suspended the finest particles of carbonate of lime and alumine 
still mixed with the finest sands. In this distribution of mate¬ 
rials, we learn that three contemporaneous deposits may go on, 
the siliceous, the conglomerate and sandstone, the clay slates and 
shales, and the limestone rocks. As marine animals and plants 
occupy different stations, some living in shallow and others in 
deep water, it is plain that these rocks which are being 
deposited, will very likely contain the remains of animals of 
different species, though of contemporaneous formation. The 
nature of the bottom, too, influences the law of distribution of 
animals, as well as plants. Hence, at the same depth, and 
under other circumstances which are equal, a sandy shore is 
* Conglomerates are cemented beds of pebbles formed in water. Pudding 
stones are cemented pebbles formed above water upon dry land, by percolation 
of water holding carbonate of lime and silex in solution. So the travertin and 
tufa are formed above water. 
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