98 HISTORY OF THE EARTH. 
commensurate to the quantity of soil which would he necessary 
to fill up the vallies in the interior. Other objections might be 
proposed. Cases occur where there is such a combination of 
longitudinal and transverse vallies, as proves incontestibly, that 
the bed of the river was first formed, and the water afterwards 
flowed in it, in obedience to the law of gravity, but without exert- 
ing any considerable agency in producing its excavation. The 
Shenandoah and Potomac rivers in Virginia, will furnish an il- 
lustration^ though they they may not afford an example. Mr. 
Jefferson supposes that the Blue Ridge was first thrown up, that 
the two rivers afterwards began to flow, and to form a lake be- 
hind the mountain, which continued to rise, until it reached the crust 
of the ridge, when it broke over, and gradually tore away the 
strata down to the present level of the bed of the stream. This 
may be a correct account of what took place at the point where 
the Potomac passes the ridge, but cases occur on the surface of 
the globe, where the appearances are the same, so far as relates 
to the rupture of the mountain barrier, but it is found on exami- 
nation, that the crest of the mountain is higher than the head of 
the river, or than some other point on the edge of the basin, within 
which the waters are supposed to have been confined, before they 
broke through : and if the elevated land which separates the head 
waters of the Shenandoah from the waters of James River, shall 
be found to be lower than what was the original gap in the Blue 
Ridge at Harper's Ferry, or if any other point in the ridge, shall 
turn out to be lower than the same gap, it is evident that the pro- 
posed explanation of the appearances will be inadmissible. We 
must then resort to some of those primeval currents that have 
changed the face of other countries. 
The existing vallies may be referred to three principal causes. 
1. The irregular elevation and subsidence of the rocky stra- 
ta of the globe, which have produced the greater inequalities of 
surface. 
2. Currents established in the ocean whilst large parts at least 
of the present continents were covered with water. By these 
the secondary strata have been torn away in places, and over large 
areas, to a depth of some hundreds of feet, and the materials car- 
ried off, creating what are called denudations ; the inferior beds 
having been uncovered and brought up to the surface. Of this, 
the district between London and Brighton on the southern coast 
of England furnishes a remarkable example. When the effect is 
such as to create a depression below the general surface of the 
country there is formed a valley of denudation. 
3. Rivers. These are constantly, though sometimes very 
slowly acting upon their beds, and changing the form and aspect 
of the vallies through which they flow, "it is evident that those 
depressions in their channels in which lakes are formed, must 
gradually disappear, being partly filled by the alluvion brought 
in at their upper, and partly drained by the wearing away of the 
