A CENTURY OF GEOLOGY. 273 



the lifted strata broke along the top of the arch, and melted matter 

 was forced through between the parted strata, i)ushino- them hack and 

 folding them on each side. And hence the typical form of mountain 

 ranges is that of a granite axis along the crest and folded strata on 

 each Hank. But attention has lateh' been drawn to the fact that some 

 mountains, as, for example, the Appalachian, the Uintah, etc., consist 

 of folded strata alone, without any granite axis. In such ranges it is 

 plain that the whole height is due not to any force acting from below, 

 but to a lateral pressure crushing and folding the strata, and a corre- 

 sponding thickening and bulging of the same along the line of crush- 

 ing. Then the idea was applied to all mountain ranges. So soon as 

 the prodigious amount of erosion suffered by mountains, greater often 

 than all that is left of them, was fully appreciated, it became evident 

 that the granite axis so characteristic of mountains was not necessarily 

 pushed up from beneath and protruded through the parted strata, but 

 was in man}' cases only a submountain core of igneous matter slowh^ 

 cooled into granite and exposed by subsequent erosion greatest along 

 the crest. 



Next, attention was drawn to the enormous thickness of the strata 

 involved in the folded structure of mountains. From this it became 

 evident that the places of mountains before they were formed were 

 marginal sea bottoms off' the coasts of continents, and receiving the 

 whole washings of the continents. Thus the steps of the process of 

 mountain formation were (1) accumulation of sediments on offshore 

 sea bottoms until by pari passu subsidence an enormous thickness was 

 attained. This is the preparation. (2) A yielding along these lines 

 to the increasing lateral pressure with folding and bulging of the strata 

 along the line of yielding, until the mountain emerges above the ocean 

 and is added to the land as a coast range. This is mountain birth. 

 (3) As soon as it appears above the water it is attacked })y erosi\'e 

 agents. At first the rising by continuance of the crushing and ])ulging 

 is in excess of the erosion, and the mountain grows. This is mountain 

 youth. (4) Then supply and waste balance one another, and we have 

 mountain maturity. (5) Then the erosive waste exceeds the growth 

 by upbulging, and mountain decay begins. (6) Finally, the erosive 

 forces triumph and the mountain is clean swept away, leaving only 

 the complexly folded rocks of enormous thickness to mark the phice 

 of a former njountain. This is mountain death. Such, briefly, is the 

 life history of a mountain range. 



In all this we have said nothing about causes. In this connection 

 there are two points of especial importance: (1) Why does the yield- 

 ing to lateral pressure take place along lines of thick sediments^ 

 (2) What is the cause of the lateral pressure ? 



