316 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



§ 16. Hozv a sea valley develops and gives rise to parallel moun- 

 tain chains, as illustrated in the San Joaquin, betzveen the Sierra 

 Nevada and Coast Range, in California. 



We have already seen that mountains are raised by the injection 

 of the coast by steam-saturated lava exploding beneath the earth's 

 crust. To understand the entire working of this process under good 

 conditions, we might study the different sea valleys now existing in 

 various parts of the world, or take one which shows the characteris- 

 tic features of the process. If we could find one in which the process 

 is complete, but of recent date geologically, its present form would, 

 no doubt, enable us to make out the transformation which is under- 

 gone at different stages. The Sierra Nevada mountains, with the 

 adjacent San Joaquin valley, appear to be an ideal case to illustrate 

 the process in question. 



A study of the Sierras in California shows that the western 

 slopes of these gigantic mountains are very gradual — less than 

 one in fifty — and braced by many spurs, with deep intervening 

 cafions, of which Yosemite is the most famous. The eastern slopes 

 of the Sierras are about ten times steeper than the western, and the 

 jutting spurs are largely wanting. This shows that the injecting 

 forces which raised these mountains came almost entirely from the 

 west ; and they continued so long that they finally gave the range an 

 unsymmetrical form^ — gently sloping and deeply corrugated with 

 cafions on the west, and steep and precipitous on the east. 



The form thus taken by the Sierras would indicate that at a 

 late stage in their history the San Joaquin valley took the form 

 shown in the accompanying figure, long and sloping on the east and 



* The more gradual slope of a mountain range toward the sea is due to 

 two causes more or less distinct: (i) The vertical upheaval of the chain, 

 with the successive horizontal thrusts which push it little by little from the 

 sea, thus jiaturally making the farther slope the steeper; (2) the subse- 

 quent elevation of the shore, by injections under the crust, which tips the 

 range still further over, by raising the base of an incline that is already 

 gradual. These two causes combined will be found to explain the principal 

 inequalities in the slopes of mountain ranges as observed in different parts 

 of the world. When the range is being first upheaved, the injections give 

 a nearly vertical uplift; as the range gets older the injections come more 

 and more from the seaward side ; when the range itself is finished, the 

 injections only tip its base upward, and make the seaward slope more and 

 more gradual. 



