HOW MOUNTAINS ARE FORMED — LYTTLETON 359 



and the processes will automatically always find the weakest parts of 

 the planet's crust. 



It is unlikely that the mountains were produced exactly in their 

 present forms. Long ridges would develop where one layer was thrust 

 over another, and then erosion would carve out gorges and canyons 

 by wearing away huge quantities of the more readily removable ma- 

 terial {see pi. 2). The resulting reduction in weight would cause the 

 whole area gradually to rise, maintaining a kind of floating equilibrium 

 on the layers below. This would increase the surface irregularity, 

 though clearly there is a limit to which the process could go. Simi- 

 larly, where the relief of stresses took the form of folding of the sur- 

 face layers, subsequent erosion would accentuate the surface features, 

 at least for a time. 



THE EARTH'S STORY IN OUTLINE 



Thus it now seems probable that the Earth began as a cool feature- 

 less planet with minute traces of radioactive minerals spread through 

 its volume. Aeons may have passed while the internal temperature 

 slowly but inexorably rose, until suddenly the crucial melting point at 

 the center was reached and the process of contraction was set in motion. 

 Compression of the liquefied central part would take place automati- 

 cally at this stage because of the high pressure, and the outer parts 

 would then follow down to restore equilibrium. Continuing compres- 

 sion would begin the cycle of mountain formation by building up 

 stresses in the outer layers. This would be followed eventually by 

 catastrophic release as the surface rocks folded and fractured, and 

 erosion of the resulting foldings and thrustings would finally produce 

 huge areas of mountain ranges. And there is no reason to suppose that 

 the process has ceased : the lifetimes of radioactive elements are such 

 that heat is still being produced throughout the Earth, though cer- 

 tainly at only a fraction of the original rate. But until it practically 

 ceases altogether, the Earth will go on contracting and periods of 

 mountain building will continue to occur. 



MOUNTAINS ON OTHER PLANETS? 



To the question : could mountains be formed by this process on the 

 moon or on any of the small planets, such as Mercury, Venus, or Mars, 

 the theory can in fact give quite definite answers. Venus, for example, 

 has an observed radius consistent with the value it would have if the 

 planet is made of material with similar properties to the Earth. Since 

 its mass is only a little less than that of the Earth, the internal condi- 

 tions of pressure and temperature are likely to be such that melting 

 near the center has occurred, and a liquid core formed deep within it, 

 but not to quite the same extent as in the Earth. Folded and thrusted 

 mountains would therefore be expected to be found on Venus. 



