114 GEOGNOSY AND GEOLOGY. 
have shattered mountains, where showers of ashes have buried cities, and 
earthquakes have paralysed whole nations with terror, there it is that 
nature cannot so readily erase the traces of such catastrophes. In such 
ways changes of original condition may occur, leaving a very definite 
character. These changes are to us the hieroglyphics which describe the 
past history of our planet, and the unriddling of which is the business of the 
geologist. He indicates the causes, the geognosist only the effects. Causes, 
however, are known by their effects, and for this reason the study of the 
latter must precede that of the former. As in the investigation of any 
object the exterior must first be subjected to examination, before the internal 
peculiarities can be studied, so we but act according to sound reason in 
going first into the consideration of the exterior of mountain masses, and 
then into that of their interior, the structure, and the constituents. This 
spheroid on which we live, and whose polar flattening amounts to ;1,, 
possesses an average density of 5.67; or in other words, its density is 5.67 
times that of pure water. The mean density of the earth’s crust is, 
however, but 3.0; it must consequently increase towards the centre, and 
become greater than 5.67. If we assume that part of the earth whose 
density is 3.0, to extend to a depth of one fourth its radius, then the density of 
the interior must exceed that of wrought iron, or be more than 7.7. 
It has been calculated that nearly three fourths of the surface of our planet 
are embraced by the sea level. All above this level is called land, all below 
it sea. The rise of the land above this level is found to increase with the 
distance from the sea, forming the general elevation, the ever descending 
bottom of the sea constituting the general depression. These general 
elevations and depressions bear the same relation to our whole planet that 
special elevations and depressions do to limited tracts. It is the alternation 
of mountain and valley which modifies the continent, as also the bottom of 
the sea. Pl. 53, fig. 9, is a submarine section of the Straits of Gibraltar, 
fig. 10 a section taken between Tarifa and Alcazar on the Spanish coast. 
1. Mountains. 
The height of mountains, as well as the depth of valleys, varies to an 
extraordinary degree. It is man, not nature, who limits the almost imper- 
ceptible transition from the low plain to the highest mountain, by his 
artificial definitions. As, however, it is necessary to have some standard 
of comparison, it is customary to call an elevation of 100 feet or less, a 
hill; one under 3000 feet, a low mountain; one under 6000 feet, a mountain 
of medium height. Anything beyond this last limit is known as a high 
mountain. In measuring the heights of mountains it becomes necessary to 
determine the length of a line supposed to be let fall from the summit to the 
extended level of the sea. This line evidently expresses the relative heights 
of mountains, or their respective heights above the level of the sea. The 
relative height must be distinguished from the absolute, or that of a 
mountain from its summit to its base. For measuring heights of mountains 
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