GEOGNOSY AND GEOLOGY. 115 
the theodolite is the most appropriate instrument, being capable of a very 
accurate determination of angles. If a station be selected from which the 
summit of the mountain in question can be observed, and the angle measured 
which the line of direction to the summit makes with the horizontal, and 
the horizontal line be measured towards the foot of the mountain, and the 
angular elevation of the top again taken from the other extremity, then we 
shall have all. the data necessary to a trigonometrical determination of the 
point in question. The more usual instrument for measuring heights is, 
however, the barometer, the mercury in which stands at a different elevation 
with every difference in the distance from the level of the sea. The 
apparent simplicity of this method is nevertheless affected by several 
modifying causes, as the amount of moisture in the air, the temperature, the 
character of aerial currents, &c. 
Every mountain may be divided into the top, summit, or head; the 
middle, face, or body; and the bottom or foot. The plane on which the 
foot is supposed to stand is called its base, and the faces of the mountain 
are formed by the declivity or slope. 
The mutual relations in which the single parts of the mountain stand to 
each other determine its form, and of this we distinguish two principal 
types. Mountains proper are those whose length and breadth are pretty 
nearly the same, mountain ridges those whose length considerably exceeds 
the breadth. True mountains exhibit considerable diversities in their 
external forms; sometimes they resemble a segment of a sphere or 
paraboloid, sometimes a bell, a cone, or a pyramid. These various forms 
are not so uninteresting as might at first be supposed; the external 
appearance in itself may not, indeed, indicate any fact for geognestical 
consideration, it may, however, illustrate the peculiar relations existing 
between external form and the kind of rock. The generalization has beer. 
made, that the same rock species, when in not too inconsiderable quantity 
has constant external features, so that a practised eye may, in many cases 
draw an accurate inference as te the character of a mountain from a far 
distant view of it. Thus granite generally assumes the form of a spherical 
segment, trachyte that of the bell, while voleanic masses occur in the shape 
of acone. The differences which exist amongst mountain ridges may have 
reference either to the ridge itself, or to the vertical cross-section. In the 
first point of view we distinguish between straight and curved ridges; in 
the second, between a circular, a parabolic, and a roof-shaped cross-section. 
In considering the slope of a mountain the geognosist first investigates the 
angle which it forms with the horizon. This angle, capable of infinite 
variation, is exceedingly dificult to ascertain, even approximately, without 
instruments, its determination being very much exposed to optical 
illusions. It becomes necessary to set artificial boundaries between the 
most frequent angular differences, and to express them by artificial 
appellations. 
There may be modifications in respect to the continuity of the declivity, 
which contribute im great measure to the character of the mountain. This 
may either be uniform and uninterrupted, or may have a stairway or terrace 
ICONOGRAPHIC ENCYCIOP£DIA.—VOL. I. 35 545 
