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12 GEOLOGICAL SURVEY 
of level, by which dry land has been converted into the bed of the ocean, and again elevated, to 
form the habitation of land animals. Numerous instances of such alterations of level have occur- 
red at comparatively recent periods. It is not many years since the coast of South America was 
elevated many feet above the surface of the ocean, over a distance of 2,000 miles. And at this 
moment the coast of Scandinavia is gradually rising, while a corresponding subsidence is going on 
upon the coast of Greenland. Such instances as these show the fallacy of the opinion that the 
elevation of land is a consequence of the diminution of the waters of the ocean; for while a dimi- 
nution would be necessary on the coast of Scandinavia, an increase or rise would be required to 
explain the subsidence on the coast of Greenland. 
VoLCANOES AND HARTHQUAKES. 
These are among the most interesting of the phenomena attendant upon igneous agencies.— 
They enable us, to some extent, by comparison, to appreciate the enormous force that may be called 
into action by subterranean heat. A volcano is a rent or fissure in the earth, through which steam, 
gases, stones, and molten matter are ejected into the air. The lava, scoriz, and ashes that fall 
around the opening, which is. called a crater, form a cone of considerable elevation. Stones are 
thrown toa great height in the air; and a rock, weighing two hundred tons, was projected to a 
distance of nine miles, during an eruption of Cotopaxi, one of the volcanoes of the Andes. 
Changes of level around volcanic centres are not uncommon, and are probably due to the expan- 
sion and contraction of the liquid mass below. That these seas of molten rock are connected over 
vast areas, is rendered highly probable by the simultaneous eruptions and alternations of action of 
distant voleanoes. Their occurrence along the line of ocean coasts is remarkable, and seems to 
indicate some connection with the dislocations and fracture of strata that occur in the parallel 
mountain chains. Those which have remained dormant since the commencement of the His- 
toric period are termed extinct volcanoes. 
Certain tremulous or wave-like motions of the earth, which often occompany sudden volcanic 
eruptions, are called earthquakes. They seem to be due to paroxismal motions, produced by some 
disturbance of the molten mass below, which are propagated like waves on the surface of water, 
communicating their motion to the solid crust of the earth, frequently to a very great distance; and, 
during their periods of greatest violence, producing the most awful results. In 1811, the earth- 
quakes which destroyed the cities of La Guayra and Caraceas, in South America, produced, in South 
Carolina, sufficient violence to cause the mud and silt in the beds of the creeks along the coast to 
recede or slide from the banks towards the channel, leaving the line of parting of the mud and 
solid matter of the banks as distinct as if the latter had been elevated one foot above its former 
level. In the valley of the Mississippi the convulsions felt were sufficient to produce important physi- 
cal changes in the face of the country—such as the formation of lakes and islands, where they did | 
not exist—some of them of great extent. For miles the banks of the river sunk eight feet below 
its former level. Frightful chasms opened and sent up water and sand to a great height. The 
directions of these chasms was about N. E. and 8. W. a fact noticed by the inhabitants, who felled 
trees at right angles with this course, and placing themselves upon the trees, they often saved 
themselves when the ground opened beneath them. 
