i9o6.] SEE— THE CAUSE OF EARTHQUAKES. 285 



of earthquakes. Rains, snows and glaciers on Mount Cotopaxi 

 ought to produce sHpping of rocks, if anywhere, because the angle 

 is steep and the material loose and unsettled. We are not aware that 

 the slipping of any volcanic cone or other similar mountain has ever 

 been observed to produce a real earthquake ; and if slipping were the 

 order of nature, we should expect some enormous slips with corre- 

 sponding tremors due to this cause near Cotopaxi, Aconcagua, and 

 other great volcanoes (especially when these mountains are shaken 

 at the times of eruption), which are not observed. 



We seem compelled therefore to abandon the theory of slipping 

 and bending of rocks^ except as producing all the time infinitesimal 

 tremors called microseisms, which very likely depend to a con- 

 siderable extent on this cause. Glaciers are known to be fluid 

 masses, and they move accordingly, though very slowly. It has 

 been shown by the writer (in Nature, 1902) that a rock such as 

 marble undergoes secular bending, and is therefore fluid ; and we 

 take it that all large rock masses are very similar in their behavior, 

 though their viscosity may be and generally is greater than that of 

 marble ; and hence if movement of mountain masses or other large 

 rocks take place, it would seem that, wherever possible, it should 

 be by a very gradual yielding. The cases in which very large 

 masses of rock, like the sides of a mountain, acquire such unstable 

 positions as to fall, do not seem to be very numerous.^ Accordingly, 

 it is difficult to believe that this cause is very effective in producing 

 earthquakes ; for such shocks as might result from it would be rare, 

 small and unimportant. And moreover they could never occur 

 where the average slope is anything like so small as i in 20. Be- 

 sides the arguments here outlined there is another hardly less effec- 

 tive which w^e shall merely mention, namely : That the forces which 

 may shake an entire continent and send waves of compression and 



^ This hypothesis was originally proposed by Boussingault, from observa- 

 tions made on earthquakes noticed in the Andes remote from known vol- 

 canoes, and has at length developed into the tectonic theory now widely 

 held by seismologists and geologists. 



^The movement by sliding of one or two mountains in the Alps is 

 recorded within the historical period. Among the Andes the most noted 

 change is the collapse of the crater of Carihuairazo, adjacent to Chimborazo, 

 during a violent earthquake on the night of 19-20 of June, 1698. Before 

 this disaster Carihuairazo is said to have been taller than 'Chimborazo. 



