CAUSES OP GEOLOGICAL CHANGES. 67 
low the general level of the country and below the level of the 
sea. It is to this ridge that the plantations near the river, both 
above and below New Orleans are confined. It is evident that 
after having observed the rate of yearly increase, and traced the 
alluvial formation up to the point where it commenced, we have, 
as in the case of the trap rocks, the data necessary for calculating, 
though not with any very great degree of accuracy, the time when 
it began to be created. 
In some countries, springs rise out of the earth charged with car- 
bonate of lime, which they hold dissolved by means of the carbonic 
acid they contain, and others are impregnated silica. These sub- 
stances they deposit, commonly at no great distance from the 
fountain head. The travertino of the Roman states, extensively 
used as a building stone, and the incrustations about the hot springs 
of Iceland and the Azores, are examples of such formations. 
3. The sea; which in some p arts of the world is rapidly un- 
dermining the coasts of the continent, and in others throicing 
up lo?7g ranges of sand banks. 
Where it is the former process that is going on; the earth that 
falls down when the under-stratum that supported it is removed, 
is first accumulated around the base of the cliff. Afterwards the 
influx and efflux of the tide spreads this mass over the bottom of 
the adjacent sea, till at length a bank is formed which curbs the 
fury of the waves, and prevents any farther encroachment. But 
when a current sweeping over the spot carries off the materials of 
the cliff as they fall, the work of destruction goes on indefinitely. 
The sea is in this way making inroads upon the land at a great 
number of points on the eastern coast of England. 
In the other case the flood tide brings in a quantity of mud and 
sand, which the succeeding ebb does not carry away, and a long 
line of banks is sometimes formed at a distance from the ancient 
boundary of land and water. It is in this way, according to some 
geologists, that the whole low country of the United States has 
been thrown up. 
4. Ji very diminutive race of animals — the Zoophytes which 
inhabit the various kinds of coral and madrepore, are form- 
ing islands in the mid-ocean, and reefs and shelves along the 
coasts of the continents . 
The Keel Sea, the Persian Gulf, the Indian Ocean and that part 
of the Pacific lying within the latitude of thirty degrees on 
both sides of the equator, are amongst the well known theatres of 
their operations; the activity and amount of which, however, ap- 
pear to have been over-rated. Thus the harbours of the Red Sea, 
upon the excellence of which, as well as upon the safety of the 
navigation of the sea itself, the commercial prosperity of Ancient 
Egypt depended, have been represented as entirely choaked 
up by reefs of coral that have been created in later times, and are 
still increasing in extent. But the ruin of these harbours is now 
