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 by erosive agents. At first the rising by continuance of the crushing and bulging 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 up-bulging, 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 place of a former mountain. 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 yielding to lateral pressure take place along lines of thick sediments? (2) What is the cause of the lateral pressure?
1. _Cause of Yielding to Lateral Pressure along Lines of Thick Sediments._--The earth was once very hot. It is still very hot within, and still very slowly cooling. If sediments accumulate upon a sea bottom the interior heat will tend to rise so as to keep at the same distance from the surface. If the sediments are very thick, say five to ten miles, their lower parts will be invaded by a temperature of not less than 500° to 1,000° F. This temperature, in the presence of water (the included water of the sediments), would be sufficient to produce softening or even fusion of the sediments and of the sea floor on which they rest. This would establish a line of weakness, and therefore a line of yielding, crushing, folding, bulging, and thus a mountain range. In the first formation of a range, therefore, there would necessarily be a sub-mountain mass of fused or semifused matter which by the lateral crushing might be squeezed into cracks or fissures, forming dikes. But in any case the sub-mountain mass would cool into a granite core which by erosion may be exposed along the crest. The explanation seems to be satisfactory.