MOUNTAIN ORIGIN AND STRUCTURE. 263 



may be exposed by erosion as dikes. Thus lava-floods are associated 

 with newer- strata, and dikes with all, but especially the older. 



2. Volcanoes. — Great lava-floods come up through fissures and flow 

 off as sheets. By repeated eruptions successive sheets accumulate until 

 the whole mass is several thousand feet thick. The lower parts of such 

 lava-masses remain incandescently hot almost indefinitely. Percolat- 

 ing water reaching these hot interior portions develops force sufficient 

 to eject fused matter and form volcanoes parasitic on the lava-floods ; 

 or else water may reach the sub-mountain liquid through the fissures 

 produced by foldings and thus also produce volcanoes. Thus volcanoes 

 also are associated with mountain-ranges. 



3. Mineral Veins. — If the fractures do not penetrate deep enough 

 to reach the sub-mountain liquid, then they are not filled at the time 

 of their formation with liquid lava, but slowly afterward by deposit of 

 mineral matter from percolating waters and form veins. Thus min- 

 eral veins are especially abundant in mountain-regions. 



4. Faults and Earthquakes. — The walls of great fissures, as we 

 have already seen, never remain in their original position, but always 

 slip one on the other and thus form faults, which, in case of foldings 

 by lateral presssure, will usually be reverse. Hence faults are associated 

 with mountains. The slipping, however, will not take place all at once 

 but very slowly, and yet not uniformly, but more or less paroxysmally. 

 Each paroxysm will produce an earthquake. The original fracturing 

 will also produce an earthquake. Thus earthquakes are associated 

 with mountains, especially if the mountains are still groiving. 



We see thus the truth of the proposition with which we set out, that 

 mountains are the theatres of the greatest activity of all geological 

 agencies. They are first the places of greatest activity of aqueous 

 agencies in the form of sedimentation in preparation for the future 

 mountains ; then of igneous agencies in the birth and growth of the 

 actual mountain; and, finally, again of aqueous erosive agencies in 

 sculpturing them into forms of beauty, but also in the decay and at 

 last in the complete destruction of former mountains. 



Cause of Lateral Pressure. — We have thus proved that the imme- 

 diate cause of the origin and the growth of mountains is lateral press- 

 ure acting on thick sediments crushing them together and swelling 

 them up along the line of greatest thickness. But still the question 

 remains, What is the ultimate cause, i. e., the cause of the lateral press- 

 ure ? This, as we have already said, lies still in the domain of doubt 

 and discussion, but the view which se,ems most probable may be briefly 

 stated as follows : 



In the secular cooling of the earth there would be not only unequal 

 radial contraction, giving rise, as shown on page 168, to continents and 

 ocean-basins, but also unequal contraction of the exterior as compared 



