336 MOUNTAIN RANGES 



cradle of the mountains. The area of the trough varies from 

 time to time, as do also the position of the line of maximum 

 subsidence and the relative rate of depression and sedimenta- 

 tion, so that the depth of water varies. We saw above that 

 the strata of mountain ranges are very much thicker than the 

 same strata in the adjoining plains, which means that the ranges 

 have been formed along the lines of maximum sedimentation. 



The second stage in the building of a range is the upheaval of 

 the thick mass of strata into a series of anticlinal and synclinal 

 folds, which may be upright, open, and symmetrical, or closed, 

 asymmetrical, inclined, or inverted. This, as we have already 

 learned, can be produced only by lateral compression, a con- 

 clusion which is sustained not only by the mechanics of fold- 

 ing and faulting, but also by the less obvious structures, such 

 as cleavage and fissility, metamorphism, the microscopic crum- 

 plings and plications, and the crushing and flowage of the mineral 

 particles. Experiments upon the lateral compression of plastic 

 substances under load give the same result. The compress- 

 ing force does not raise anticlines with great cavities beneath 

 them, for such arches could not well be self-supporting, but 

 mashes together the whole mass of strata, raising them into folds 

 and wrinkles, crowding the beds into a greatly reduced breadth ; 

 or when they are not sufficiently loaded to be plastic, break- 

 ing and dislocating them in great thrust faults. Certain mountain 

 areas in Pennsylvania have been compressed into one-sixth the 

 breadth originally occupied. It is not necessary to suppose that 

 a mountain range was thrown up by one steady movement. On 

 the contrary, there is good reason to believe that repeated move- 

 ments, separated it may be by long intervals of time, have been 

 engaged in the work. 



That mountain ranges have been forced upward by lateral com- 

 pression is an unquestioned fact, but to determine how that com- 

 pression was generated is a much more difficult problem. The 

 most satisfactory explanation yet offered is that the compression 

 is due to the contraction of the globe from cooling. The earth's 

 crust long ago reached a state of fairly constant temperature, but 



