STRUCTURES COMMON TO ALL ROCKS. 245 



thick sediments. Every one of our great mountain-ranges 

 can be shown by geological evidence, which we cannot 

 give here, to have occupied this position until the time 

 of their birth.* 



Different Stages of Mountain-Life. We have said 

 " until birth," but it must not be supposed that there was 

 anything sudden about it. The emergence above water 

 we call its birth, but a mountain continues to grow stead- 

 ily through many ages. Meanwhile, as soon as it is born, 

 erosion commences, and continues with increasing rate as 

 the range grows higher. When the mountain stops grow- 

 ing, erosion begins to destroy it, and finally levels it com- 

 pletely. Thus, in every mountain there is a period of 

 birth, a period of growtlj, a period of maturity, a period 

 of decay, and a time of death or obliteration. Many of 

 the earliest mountains have been entirely swept away. 

 We know their places only by their folded structure 

 fossil bones of extinct mountains. 



Why Yielding occurs along Lines of Thick Sedi- 

 ments. Perhaps the pupil has already asked himself, 

 " Why does yielding occur only along lines of thick sedi- 

 ments ?" The probable reason is, that great accumu- 

 lations cause the rise of the interior heat of the earth 

 toward the surface, as already explained on page 227. 

 This heat, in the presence of the water included in the 

 sediments, causes these, as also the earth-crust beneath, 

 to soften or even semi-fuse ; and thus creates a line of 

 weakness, and therefore of yielding. This is represented 

 in the experiment with the wax, on page 240, by the gen- 

 tle softening of the middle part. 



Cause of the Lateral Pressure. If it be further 

 asked, " What is the cause of the lateral pressure ?" we 

 can only say that this is an obscure point, and one much 

 discussed. It is probable, however, that it is due, as 

 already stated (page 239), to the interior contraction of 

 * For evidence, see " Elements of Geology," p. 265. 



