FRACTURES OF THE STRATA AND FAULTS. 
87 
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beds, which have been _ Fig. 73 . 
folded in the manner seen 
in Fig. 73, so that each 
of them is twice repeat¬ 
ed, the position of one 
half being reversed, and 
part of No. 1, originally 
the uppermost, having 
now become the lowest 
of the series. 
These phenomena are 
observable on a magnificent scale in certain regions in 
Switzerland, in precipices often more than 2000 feet in per¬ 
pendicular height, and there are flexures not inferior in di¬ 
mensions in the Pyrenees. The upper part of the curves 
seen in this diagram. Fig. 73, and expressed in fainter lines, 
has been removed by what is called denudation, to be after¬ 
wards explained. 
Fractures of the Strata and Faults. —Numerous rents may 
often be seen in rocks which appear to have been simply 
broken, the fractured parts still remaining in contact; but 
we often find a fissure, several inches or yards wide, inter¬ 
vening between the disunited portions. These fissures are 
usually filled with fine earth and sand, or with angular frag¬ 
ments of stone, evidently derived from the fracture of the 
contiguous rocks. 
The face of each wall of the fissure is often beautifully 
polished, as if glazed, striated, or scored with parallel fur¬ 
rows and ridges, such as would be produced by the contin¬ 
ued rubbing together of surfaces of unequal hardness. These 
polished surfaces are called by miners “ slickensides.” It is 
supposed that the lines of the striae indicate the direction 
in which the rocks were moved. During one of the minor 
earthquakes in Chili, in 1840, the brick walls of a building 
were rent vertically in several places, and made to vibrate 
for several minutes during each shock, after which they re¬ 
mained uninjured, and without any opening, although the 
line of each crack was still visible. When all movement 
had ceased, there were seen on the floor of the house, at the 
bottom of each rent, small heaps of fine brick-dust, evident¬ 
ly produced by trituration. 
It is not uncommon to find the mass of rock on one side 
of a fissure thrown up above or down below the mass with 
which it was once in contact on the other side. “ This mode 
of displacement is called a fault, shift, slip, or throw.” “ The 
miner,” says Playfair, describing a fault, “ is often perplexed, 
