440 POPULAR SCIENCE MONTHLY.. 



pressure with folding and bulging of tlie strata along tlie line of 

 yielding, until the mountain emerges above the ocean and is added 

 to the land as a coast range. This is mountain hirth. (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 connec- 

 tion there are two points of especial importance: (1) Why does 

 the yielding to lateral pressure take place along lines of thick sedi- 

 ments? (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 

 tliick, 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 sedi- 

 ments and of the sea floor on which they rest. This would estab- 

 lish a line of weakness, and therefore a line of yielding, crushing, 

 folding, bulging, and thus a mountain range. In the first for- 

 mation 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 ex- 

 planation seems to be satisfactory. 



2. Cause of the Lateral Pressure. — No question in geology has 

 been more discussed than this, and yet none is more difficult and 

 the solution of which is more uncertain. But the most obvious 

 and as yet the most probable view is that it is the result of the 

 secular contraction of the earth which has gone on throughout its 

 whole history, and is still going on. 



It is admitted by all that in an earth cooling from primal incan- 

 descence there must come a time when the surface, having become 

 substantially cool and receiving heat also from the sun, would no 



