1886.] NEW YORK ACADEMY OF SCIENCES. 23 



''throw'* or displacement sometimes amounts to many thousand 

 feet. Earthquakes, mountain chains, and volcanic eruptions may 

 all be considered as consequences of this readjustment. Moun- 

 tain chains are great lines of fracture in the earth's crust along 

 which rocks before nearly horizontal are raised into ridges by 

 lateral pressure. They have been compared, not inaptly, to the 

 wrinkles formed in the rind of a fruit when it loses its volume 

 by drying. Every mountain chain shows many folds and faults; 

 in the Alleghanies the folds are noticed by every traveller, and 

 few better examples of folded strata are seen anywhere than 

 those which border the gorge which cuts through the ridges 

 from Cumberland to Frostburg, Maryland. The faults are less 

 conspicuous and would hardly be detected except by a geologist, 

 but they are very numerous, and in some of them the displace- 

 ment is more than 20,000 feet. The Wasatch Mountains in Utah 

 — one of the boldest ranges on the continent — owe their relief 

 mainly to a fault which runs nearly north and south through the 

 middle of the Territory. The country west of this fault is 

 thrown down and that on the east raised to form a wall 5,000 to 

 7,000 feet high. 



It is evident that the folds and fractures seen in every moun- 

 tain belt could not have taken place without great disturbance of 

 the surrounding country. And as they have been formed, not 

 all at once, but each by itself, and each one by many paroxysms, 

 an almost infinite series of earthquakes is recorded in the struc- 

 ture of every mountain chain. A large number of earthquakes 

 of modern times have been attended by changes of topography 

 which have remained as evidences of the displacements which 

 caused the vibrations. Sometimes a line of coast was raised 

 above the ocean level, sometimes mountains or cliffs split and 

 fell, sometimes fissures and faults were formed many miles in 

 length. 



Another thing about mountain chains is not so generally 

 known as that they are lines or belts of folded and fractured 

 rocks ; and that is that they are the products not of moments or 

 even years, but of ages. The lines of fracture which are marked 

 by mountain chains are ever, after the first disruption, lines of 

 weakness, where the resistance to lateral pressure is diminished, 

 and where the strain of large unbroken areas is relieved from 

 time to time by displacements, necessarily attended by earth- 

 quakes. I have sometimes compared them to hinges on which 

 the great tables of the earth's crust turn with constantly chang- 

 ing angles. Generally mountain chains may be said to grow by 

 the constant or paroxysmal elevation of their arches, the increase 

 in the throw of their faults. This growth would be much more 

 apparent than it is, if it were not that the mountain chains re- 



