558 THE JOURNAL OF GEOLOGY. 



that the shore side. This thickest line, as we have seen, becomes 

 the crest, which therefore is asymmetrically placed on the land- 

 side or side from which the sediments were derived. The over- 

 folding on the contrary is to the sea-ward. 



SUMMARY STATEMENT OF THE FORMAL THEORY. 



We may therefore group all these inferences and sum up our 

 view of the mode of mountain formation thus : 



I. Mountain ranges, while in preparation for future birth, 

 were marginal sea-bottoms receiving abundant sediment from an 

 adjacent land-mass and slowly subsiding under the increasing 

 weight. 2. They were at first formed, and continued for a time 

 to grow, by lateral pressure crushing and folding the strata together 

 horizontally and swelling them up vertically along a certain line 

 of easiest yielding. 3. That this line of easiest yielding is deter- 

 mined by the hydrothermal softening of the earth's crust along 

 the line of thickest sedimentation. 4. That this line, by soften- 

 ing, becomes also the line of greatest metamorphism ; and by 

 yielding, the line of greatest folding and greatest elevation. But 

 (5) when the softening is very great sometimes the harder lateral 

 strata are jammed in under the crest, giving rise to Fan-structure, 

 in which case the most complex foldings may be near but not at 

 the crest. Finally (6) the mountains thus formed will be asym- 

 metric because the sedimentary cilinder-lenses from which they 

 originated were asymmetric. 



SOME EXAMPLES ILLUSTRATING. 



It is hardly necessary to enforce these views by illustrative 

 examples. They at once arise in the mind of every geologist. 

 But there are those in this audience who are not geologists. I 

 therefore select a few examples among our own mountains. 



1. Appalachian. It is well known that during the whole 

 Palaeozoic, the region now occupied by the Appalachian was the 

 eastern marginal bottom of the great interior Palaeozoic Sea, 

 receiving abundant sediments from an eastern land mass of 

 Archaean rocks, which then extended far beyond the present limits 

 of the continent and whose western coast-line was a little to the east 



