786 MR. W. BAELOW Olf THE HORIZONTAL 



prepares it for degradation ; and therefore the above phenomenon 

 observed by Dutton is what Tve ought to expect if the jointing and 

 Assuring produced in this manner in the cliffs of the great terraces 

 are appreciable *. 



I believe, then, that Tve have in this weakening effect of gravitation 

 on rocks an important key to some of the peculiar features of the 

 great erosion which has taken place in the plateau country of the 

 American Union, and, indeed, that it is an important factor in the 

 waste of almost all cliffs. I should not be surprised if it were found 

 to have a very appreciable influence in all cases of mountain denu- 

 dation. 



Prom the consideration of the production of joints and small, 

 comparatively superficial fissures by gravitation, I will now pass to 

 the consideration of the production of extensive fissures by the same 

 agency. 



Tn a landslip the spreading of some underlying bed, which has 

 become plastic through the percolation of water, or from some other 

 cause, drags apart the more solid intractable beds above, and produces 

 fissures and fractures transverse to the direction of movement f. 



Pamiliar examples of Assuring produced in this way are often seen 

 in railway- cuttings made through clay, also on the verge of sea-cliffs. 

 The horizontal movement which produces the open fissures is in these 

 cases, as in the case of most large landslips, due to a squeezing and 

 lateral extension of the material some distance below the surface, 

 and the consequent dragging apart of the mass above. 



I suggest that most of the fissures produced in volcanic districts 

 have a similar origin, and also that the same simple cause is the 

 origin of trap-dykes. 



First, as to the production of fissures. Wherever a considerable 

 body of molten rock exists below the surface, its own weight and 

 the weight of the solid rocks resting upon it will together produce 

 considerable hydrostatic pressure throughout the molten mass. And 

 the rigidity of the crust not being perfect, some movement, slight 

 or otherwise, of the molten matter will take place towards points 

 where the superincumbent weight is least — that is, provided there 

 is not absolute equihbrium. 



Therefore if the ground-surface is much higher over the tract of 

 molten matter than it is just beyond its limits, the molten rock will 

 tend to spread by its own weight and that of the solid crust resting 

 upon it. And as all rocks are more or less plastic, we may, in this 

 case, look for some horizontal movement, small though it be, of the 

 solid rock at the confines of the molten mass, and which is subjected 

 to its thrust. 



* Button refers the phenomenon to the fact that those regions which have 

 been elevated most have been most degraded by erosion ; but this explanation 

 does not appear to account for the lower portions of the higher cliflPs having a 

 greater rate of recession than the corresponding portions of the lower cliffs, but 

 only for the greater erosion of the upper parts of the higlier cliffs. Indeed, 

 the lower portions of the higher cliffs are manifestly more protected from erosive 

 agency owing to the greater amount of material which falls over them from above. 



t Dana's ' Greology,' 3rd edition, p. 666. 



