MOUNTAIN ORIGIN AND STRUCTURE. 261 



softening and therefore a line of weakness, and a line of yielding to 

 the lateral pressure, and therefore also a line of mashing together and 

 folding and up-swelling — in other words, a mountain-range. As soon 

 as the yielding commences we have an additional source of heat in the 

 crushing itself. It follows from this that there is or was beneath every 

 mountain a line of fused or semi-fused matter. This we will call the 

 sub-mountain liquid. This by cooling and solidification becomes a 

 metamorphic or granitic core, which by erosion forms the metamorphic 

 or granitic axis and crest of many great mountains. 



Brief History of a Mountain-Range.— The preparation of a future 

 mountain commences by the accumulation of enormous thickness of 

 sediment off a coast-line. This is the embryonic condition of the 

 range ; it is still within the womb of ocean. Next, the line of sedi- 

 ments yields to the ever-increasing lateral thrust, and the mountain is 

 born. As soon as it appears, there begin to act upon it two opposite 

 forces — one upheaving, the other cutting away ; the one interior, the 

 other exterior — which may be compared to the opposite processes of 

 supply and waste in the animal body. So long as the supply exceeds 

 the waste, the mountain grows. When these opposite processes are in 

 equilibrium, the mountain is mature. When the waste by erosion ex- 

 ceeds the supply by upheaval, the mountain has entered upon its period 

 of decay. Finally, the destructive forces triumph, and the mountain 

 is swept clean away by erosion. This is mountain-death. We find 

 mountains in all these stages. The Sierra, the Wahsatch, and the Coast 

 Eange, are probably still growing. The Appalachian is already mature 

 or probably entered on its period of decay. In the folded structures 

 of the enormously thick rocks of the Archaean region of Canada we un- 

 doubtedly find the bones of extinct mountains. 



Slowness of Mountain Origin and Growth. — Although, as we shall 

 see in Part III, the formation of mountains often marks the boundaries 

 of geological periods, and therefore, in a geological sense, the process 

 is comparatively rapid, yet in a human sense it is always extremely 

 slow — so slow that it may and probably is going on now under our 

 eyes, without attracting our attention. 



Age of Mountains. — The date of mountain-birth is determined by 

 the age of the strata. It must be later than the youngest strata which 

 enter into the folded structure of the mountain or are tilted on its 

 flanks. Thus we say that the Appalachian was born at the end of 

 the Coal period, because all the Palaeozoic strata, including the coal, 

 enter into the composition of its folded structure, but later strata do 

 not. We say that the Sierra was formed at the end of the Jurassic, be- 

 cause these are the youngest strata which are folded and tilted on its 

 flanks. Similarly, the Cretaceous, the Eocene, and the Miocene, are all 

 crumpled up in the Coast Range, but the Pliocene are not. Therefore 



