124 GEOGNOSY AND GEOLOGY. 
nation of the stratification on opposite sides; this existing in the same plane 
on. both sides of the valley (pl. 48, figs. 1 and 2). Such are called valleys 
of excavation or denudation. In pl. 43, fig. 3, the same stratum, a, is seen 
on the left-hand side much higher above the bottom of the valley, B, than on 
the right side, without any difference existing in the general direction. The 
determination of the true character of mountains and valleys from the 
nature of the accompanying stratification, however simple it may be 
theoretically, is yet very much embarrassed in practice, by the vegetation 
and surface soil which hide the subjacent rocks. In individual cases, 
however, this difficulty does not exist, and the whole problem can be solved 
at a single glance. These relations are beautifully seen in parts of the Mont 
Blane chain, whose tabularly cleft rocks are presented with vertical fissures. 
Weathering causes the loss of considerable portions, as is often the case 
with granite and gneiss. In this way steep pyramidal rock walls are left, 
which form the boldest and most singular groups. Some idea of this 
condition of things may be obtained from an examination of pl. 44, fig. 10, 
this representing a view of the Mont Blanc chain from the Breverberg. 
Here a indicates the celebrated vale of Chamouny; b, Mont Blanc; c, La 
Mer de Glace, a glacier; d, the Bosson’s glacier ; e, |’Aiguille verte ; f, le 
Dome du Gouté; g, la Montanvert. This character of rock surface 
exhibits a magnificent appearance when traversing whole ranges. The 
names of horns, needles, teeth, &c., given to the different projections by the 
inhabitants of the Alps, are derived from real or fancied resemblances. The 
vicinity of Barschwyl in the Solothurn Jura, shows beautifully the manner 
in which the character of stratification influences the external form of a 
country. The strata are entirely denuded, and their relations to the 
mountain formations is evident at a glance (pl. 44, fig. 11). Mountain 
forms become remarkably modified when rocks of different petrographical 
character alternate with each other. Harder and more durable strata, a 
( pl. 44, fig. 12), alternate with others of much softer texture, or readier 
destructibility, 6. The former remain standing in extended rock walls, 
while the latter disappear to a greater or less extent. Terraced valley 
slopes may be produced in the same manner ( pl. 44, figs. 18 and 14). 
10. Accidentai Separation of Rocks. 
ad ° 
The parts produced by accidental separation of a rock mass, or by 
fissures and joints, possess an undefined, irregular form. They are 
entirely incidental in their origin, and may either be confined to a smal} 
space, or traverse whole mountain masses. The fissures are of various 
sizes, from thin cracks to extended and sometimes wide gaps or fissures. 
Important effects sometimes accompany the formation of such accidental 
cracks and fissures. Whole mountain masses are often shattered, or 
otherwise affected, and debacles not unfrequently produced. 
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