50 G. K. Gilbert—Earthquakes of the Great Basin. 
tion gives to slow motion an uninterrupted or rhythmic 
character. 
The disagreeable jarring of a railway car while the brake is 
set is due to the interruption of motion by friction, the wheels 
alternately sliding and stopping. The musical vibration of a 
violin string is due to the alternate cohesion and sliding of the 
bow upon it, and fails when the friction of the bow is insuffi- 
cient. Attach a rope toa heavy box and drag it slowly, by 
means of a windlass, across a floor. As the crank is turned, 
the tension of the rope gradually increases until it suffices to. 
overcome the starting friction, as it is called. Once started, 
the box moves easily, because sliding friction is less than 
starting friction. The rope shortens or sags until its tension is 
only sufficient for the sliding friction, and it would continue in 
that state but that the box, having acquired momentum, is 
earried a little too far. This slacks the rope still more, and the 
box stops, to be started only when the tension again equals the 
starting friction. In this way the box receives an uneven, 
jerky motion. 
Something of this sort happens with the mountain. The 
upthrust produces a local strain in the crust, invelving a certain 
amount of compression and distortion, and this strain increases 
until it is sufficient to overcome the starting friction along the 
fractured surface. Suddenly, and almost instantaneously, there 
is an amount of motion sufficient to relieve the strain, and this 
height was somewhat recent. 
Let us look a moment at this evidence. The material eroded’ 
from a mountain by the elements is washed out through the 
cafions and deposited in the adjacent valleys. The coarser 
part of it lodges at the mountain base, and is built into a 
