216 SEE— FURTHER RESEARCHES ON 



[April 24, 



The accompanying figures exhibit: (i) A general section of the 

 Alps from Zurich to Lake Como (Heim and Prestwich), and (2) a 

 section on a larger scale of a portion of the central Alps (from 

 Heim's Gehirgshildiing) with fan-shaped folds and inversion of 

 strata on the two sides. It can hardly be assumed that these illus- 

 trations are extremely accurate, but no doubt they are free from 

 large errors in exhibiting the general character of the folding, which 

 gives here and there fan-shaped structures with overturned dips at 

 the sides. 



Now the explanation of such structures is the most difficult prob- 

 lem heretofore presented to the geologist. They exhibit conspicuous 

 lateral and vertical movements which cannot well be accounted for by 

 the contraction theory. A shortening of about 74 miles (Heim) 

 in the folding, which has amounted to 50 per cent, of the whole span 

 of crust (Leconte), can not be accounted for, on the old theory, 

 without assuming that the crust is loose from the globe, so that a 

 vast amount of slack could be brought forward and concentrated in 

 the folds at one point, in the Swiss Alps. This is clearly unthinkable. 

 On the other hand, the cone of matter underlying the Alps with 

 vertex at the center of the earth could not be sufficiently condensed 

 to give the required slack in the overlying crust without increas- 

 ing the density of the cone by 50 per cent., which could easily be 

 detected by geodetic observations, owing to the resulting deviations 

 of the plumb line. Accordingly we may feel sure that the matter 

 under the Alps not only is not denser than the average, but actually 

 lighter, by an appreciable amount. The crumpling of the Alps 

 cannot therefore be due to condensation beneath these mountains. 



How then did the folding arise ? 



H we cut a section across the Aleutian Islands perpendicular to 

 the chain and the parallel trench lying to the south, we shall have a 

 figure something like that shown in figure i of the following plate. 

 Now in the paper on the " Cause of Earthquakes " (§ 16) we have 

 shown how the undermining of the sea bottom sinks the trough down 

 deeper and deeper, and as the expulsion of lava continues it eventu- 

 ally becomes easier to fold up the side of the trough towards the 

 ocean (at h) and make another range of mountains parallel to the 

 first. And there is nothing to prevent the process from being re- 



