518 Geological Society : — 



of a slight elevation of the unloaded strata within the denuded ellip- 

 tical area known as the "San Kafael Swell." After alluding to 

 Dutton's suggestion that the phenomenon referred to is analogous to 

 the action of creeping in deep mines, the author discussed the nature 

 of such " creeps," which he defined as the thickening of the parts of 

 beds from which a load of superincumbent rock has been lifted, 

 caused by a thinning of the adjoining parts which remained loaded, 

 some of the substance of the latter having been squeezed out to 

 furnish the material for the thickening, and suggested that some 

 of the subsidiary plications found on the flanks of mountains are 

 caused by the thrusts arising from creeps. He also paralleled the 

 fissures in the precipices of the Grand Canon District with those 

 produced in the pillars of coal owing to the strain induced by the 

 slight inequality in the yielding of the bed supporting it, and pointed 

 out how such fissures would facilitate denudation, giving instances 

 recorded by Dutton, and that an appreciable influence might be 

 thus produced in all cases of mountain-denudation. 



The author next considered the case of a body of molten rock 

 below a considerable mass of solid rock. The pressure upon the 

 molten mass would cause movement to take place towards the point 

 where the superincumbent weight was least, provided that absolute 

 equilibrium did not exist. The overlying rocks being more or less 

 plastic, some horizontal movement of the solid rocks at the confines 

 of the molten mass, and subjected to its influence, might be looked 

 for. Any such yielding would tend to draw apart the solid crust 

 resting upon the molten rock, and the ground would open along lines 

 of weakness, such as would be produced by the presence of joints, 

 the crust in some cases breaking up into larger or smaller fragments. 



When a large mass of molten matter occurred near the surface, 

 and a fissure was produced in the way described, the weight of the 

 ruptured crust would, if the plastic mass beneath were sufficiently 

 liquid, cause the latter to rise in the fissure, producing dykes. 

 Attention was called to the fact observed by Dutton that basaltic 

 vents frequently occur on the brink of cliffs, but never at their 

 bases ; also to the existence of dykes having a strike parallel to 

 the Colorado River. In most cases the vertical fissures which 

 received the molten rock would begin to open from below, and the 

 upper strata might altogether escape rupture. 



The author discussed the case of the Henry Mountains, and 

 explained the formation of flat-topped and flat-bottomed dykes 

 according to his views. He next called attention to the influence 

 which the motions of the rocks had exercised in determining direc- 

 tions of drainage when fissures left unfilled became occupied by 

 streams. He next alluded to river-valleys, the existence of which 

 had been accounted for b3 r " antecedent " and " superimposed " 

 drainage, and suggested difficulties in the way of accepting the ex- 

 planations hitherto advanced, and considered them to be instances of 

 Assuring produced by movements of the strata due to the pressure 

 of a mass of molten or highly plastic rock spreading laterally. 



After treating of the formation of faults with normal hade, which 



