GEOGNOSY AND GEOLOGY. by 
are grouped concentrically, as also the mountain heights, single portions 
shooting up here and there independent of the rest ; plateaux or planes, often 
inclosed by the highest mountain peaks, and at a great elevation above the 
level of the sea, passes, elevated extents of land lying between mountains 
connecting opposite slopes, and thus producing a saddle form. A pass of 
this kind occurs on the St. Gothard, lying 6390 feet above the level of the 
sea, and bordered on both sides by mountains over 9000 feet in height. 
Here belong also the plains found on slopes of the mountains, as also the 
spurs which separate and run out from the body of the system. 
Mountain crests are very generally (especially in mountain chains) ranged 
one after the other, thus giving rise to a narrow linear extension, called a 
mountain range. Such ranges occur in greater or less number in the same 
mountain system; they generally run out from one, more rarely from two 
or more primary ranges, these latter being then parallel to each other. In 
mountain chains the primary range is called the longitudinal, from which 
run out the lateral or terraced range or spur. We must also distinguish 
secondary ranges from the tertiary ranges. The ranges in mountain groups 
have generally a radiated direction. The height of the secondary ranges 
usually decreases with the distance from the primary range; this, however. 
is not always the case. 
The connexion of mountain systems, when such exist, may be effected 
either immediately or indirectly. Mountainous or hilly land is generally 
the link which effects the union; it is thus between the Hartz and the 
Thiringerwald, between the Alps and the Chain of Jura. Where the 
outposts of one mountain system extend their arms into the valleys of 
another, the alliance is immediate; this is the relation between the Alps 
and the Appenines. Just as the forms of mounts and mountains are 
exceedingly various, so is it in respect to their external features. Plane 
surfaces alternate with those that are hilly, rough, and full of cavities ; 
steep rocks with deep fissures. 
3. Valleys. 
As valleys are produced by mountains, it is natural that the peculiarities 
of the former should depend on those of the latter. At first glance into a 
valley two features are readily recognised: one of these the bottom, and the 
other the walls or sides, produced by the inclosing mountains. If we 
suppose a valley to be intersected by a plane at right angles to its axis, 
many diversities will be observed in this cross-section. The bottom of the 
valley is either straight or curved, occurring both convex and concave. In 
respect to longitudinal extension, valleys are either horizontal or inclined at 
various angles. An interesting phenomenon is presented by the successive 
descents in valleys, seen particularly in the transverse valleys (those which 
intersect the longitudinal valley of a mountain chain nearly at right angles). 
The occurrence of many waterfalls is intimately connected with this 
feature in valleys. Valleys are sometimes completely inclosed by mountains, 
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