144 GEOLOGY. 
submerged and acted on by heat and pressure for ages, would become 
coal, in every particular the same as that we daily use for fuel. 
Thus, organic life of all kinds is everywhere busy in forming rock- 
masses, identical in character and appearance with those presented to our 
investigation in limestone, chalk, and coal. 
Agencies in the Alteration of Rocks. 
Rocks are changed from their original position, form, and structure by © 
two classes of agents—those that disturb, and those that wear or disin- 
tegrate. 
Disturbing Agents.—Stratified rocks in their natural state would be 
horizontal, or only slightly inclined. How, then, are we to account for the 
tiltings, upheavals, faults, and various dislocations so prevalent among the 
strata? The igneous forces, just spoken of, furnish the solution. The 
whole globe is subject to convulsive movements from the motion of the 
interior molten matter of the earth, which are seen in earthquakes, 
and by which the ground is torn into fissures, and the solid crust 
made to move in mighty undulations, that destroy and swallow great 
cities. Extensive tracts are also sometimes suddenly raised or depressed. 
Sometimes, too, great yawning craters open where previously volcanic 
movement was unknown, and continue for a time in active eruption. 
In these upheavals and subsidences, sudden or gradual, of extensive 
tracts, we see the causes at work of the dislocations of the rocks of 
former times, and of the elevations and depressions that occurred | 
throughout the geologic eras. 
Again, we know that in order to the deposition of strata of any thick- 
ness, the sea-bottom must have gradually subsided: does any such gradual 
subsidence take place at the present time? It is ascertained, from extended 
observations, that on the northern shores of the Baltic, for instance, there 
has been a gradual rise at the rate of 4 feet in a century, and in South 
America a rise of 85 feet during the human period, and at Valparaiso of 
19 feet in 220 years; while over all the world, and even round our own 
coasts, ancient sea-beaches may be seen at various elevations, marking 
former sea-levels. On the other hand, the south coast of Sweden, the 
coast of Greenland over 600 miles, and parts of South America for the 
last 300 years, have been slowly sinking ; nor are the British shores free 
from such oscillations. 
Thus, again, we see that existing causes perfectly explain the gradual 
subsidences necessary to the formation of the rocks, and to their subse- 
quent elevation into dry land. 
Disintegrating Agents——Every stratified rock in the immense thick- 

