CAUSES OP GEOLOGICAL CHANGES. 65 
of value, and assist us in explaining the past The causes of 
geological phenomena, now in active operation, are the following: 
38. 1. — The atmosphere; including the different substances 
accidentally mixed with it, and the agents by which its own 
condition and its effects upon the other forms of matter are 
modified. 
There is good reason to believe that much of the surface of the 
existing continents where it comes into contact with the atmos- 
phere, was originally a solid rock, which by the combined action 
of the air, of heat, cold, moisture, and the mechanical force of vio- 
lent storms, has been gradually disintegrated and converted into 
soil. The amount of effect produced upon the rocks by these 
destroying agents, depends upon their position, form, and structure, 
as well as their composition. The progress of these changes in 
the trap rocks is a subject of considerable interest, by reason of 
its furnishing data, from which to calculate without certain limits, 
the time when the existence of a given mass upon the surface of 
the earth commenced, and the number of the ages, therefore, 
during which the part of the earth's surface immediately about it, 
has remained pretty nearly in the condition in which we now 
behold it. 
These rocks often present themselves under the form of moun- 
tain ranges of considerable elevation and many miles in length, 
with mural precipices on one side, and sometimes on both sides. 
They are traversed by rents and fissures in different directions, 
which subjects them, especially in high latitudes, to a pretty rapid 
disintegration. Rain water insinuates itself into their crevices, 
and being congealed and expanded there, fragments of considera- 
ble size are torn off, and accumulated around the base of the 
precipice, forming at length a considerable mass reposing in a 
sloping position against the side of the mountain. What is here 
stated as true of the trap is found to obtain in a greater or less 
degree in other rock formations. 
Of the rapidity with which the process is going on, we are 
able to judge with a considerable degree of accuracy from what 
occurs from year to year, and presuming that the rate of disinte- 
gration has been in all ages pretty nearly the same, we may evi- 
dently calculate within certain limits, the time when the moun- 
tain assumed its form, and exhibited a perpendicular face from top 
to bottom. - 
Loose sands are in some parts of the world raised by the winds 
out of their bed, and driven onwards over fertile fields which are 
buried up and consigned to sterility and desolation. Thus a 
considerable portion of ancient Egypt, a soil once occupied by a 
very dense population, has been long since buried under the sands 
of the great Lybian desert. Similar effects, but on a more limited 
scale, have been produced in that part of the kingdom of France, 
