274 A CENTURY OF GEOLOGY. 



1. Cmi><(i of yielding to lateral pressure along lilies oftJi ick sediments. — 

 The earth was once very hot. It is still very hot within, and still very 

 slowl)^ cooling. If sediments accumulate upon a sea bottom the inte- 

 rior heat will tend to rise so as to keep at the same distance from the 

 surface. If the sediments are ver}^ thick, say 5 to 10 miles, their 

 lower parts will be invaded b}' 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 yield- 

 ing, 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 submountain mass would cool into a granite core which 

 by erosion may be exposed along the crest. The explanation seems to 

 be satisfactory. 



2. Cause of laterxd jjressure. — 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 w^hich has gone on throughout the whole history, and is still 

 going on. 



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

 cence there must come a time when the surface, having become sub- 

 stantially cool and receiving heat also from the sun, would no longer 

 cool or contract, but the interior, being still incandescently hot, would 

 continue to cool and contract. The interior, therefore, cooling and 

 contracting faster than the exterior crust, the latter following down 

 the ever-shrinking nucleus, would be thrust upon itself by a lateral 

 or tangential pressure which would be simply irresistible. If the 

 earth crust were a hundred times more rigid than it is, it still must 

 yield to the enormous pressure. It does yield along its weakest lines 

 with crushing, folding, bulging, and the formation of mountain ranges. 



This is the barest outline of the so-called "" contractional theory of 

 mountain formation." Very many objections have been brought 

 against it, some of them answerable and completely answered, but the 

 complete answer to others nmst be left to the next century. Perhaps 

 the greatest objection of all is the apparent insufficiency of the cause to 

 produce the enormous amount of folding found not only in existing 

 mountains but in the folded structure of rocks where mountains no 

 longer exist. But it will he observed that 1 have thus far spoken only 

 of contraction b}' loss of heat. Now, not only has this cause been 

 greatly underestimated b}- objectors, but, as shown ])v Davison and 



