CHAPTER VII. 



THE RELATIONS BETWEEN FAULTS, OPEN JOINTS, AND 

 THE FORMATION OF VALLEYS. 



BUT for the existence of faults, joints, and other shrink- 

 age fissures, very few, if any, valleys could have ac- 

 quired their present form. Those that could be formed 

 without their aid are valleys due solely to the strata 

 being bent into synclinal curves. Such valleys may 

 exist, but they are of very small extent and of rare 

 occurrence, on account of the operations which have 

 formed the features of the earth's surface. Prof. 

 Le Conte's description of the causes to which are 

 aue the elevation of mountains may be thus epitom- 

 ised. 1 Off a shore line masses of sediment gradually 

 accumulate, and their weight causes a slow subsidence ; 

 but when the accumulation of sediment becomes of 

 sufficient thickness there follows a rise of the geo- 

 isotherms, and an invasion of the sediments by the 

 interior heat of the earth. A temperature of 800 is 

 sufficient to produce, not only metamorphism, but 



i "Formation of the Features of the Earth's Surface." The 

 American Journal of Science and Art. Third Series. Vol. iv., pp. 

 460, et seq. 



