DCTTOX.l 



AGE OF THE CHASM. 121 



Another question which the geologist asks here is, how happens it that 

 the outer chasm is so broad while the inner one is so narrow? The 

 outer chasm is five to six miles wide and 2,000 feet deep; the inner is 

 about 3,500 feet wide and 3,000 feet deep. The disparity is great. We 

 have seen enough to say at once that the widening of the outer chasm 

 was effected by the recession of its cliffs. If the corrasion of the canon 

 went steadily onward without a halt or respite this disparity demands 

 some explanation. Although we should expect less recession in the cliffs 

 of the inner gorge than in those of the outer, we should not expect it to 

 be so much less if the only variable concerned was length of time. We 

 might explain it by assuming the rocks of the inner gorge to be much 

 more obdurate than those above. This is true in part, but still the dif- 

 ference in this respect is insufficient. A much more satisfactory ex- 

 planation is found in the supposition that the broad esplanade of the 

 canon between the upper palisades was an ancient base-level of erosion 

 (page 101). We might imagine that when the Colorado had cut its 

 channel down to that level, it had reached the limiting depth of corrasion 

 for the time being. Then for a long period the palisades on either side 

 wasted and receded from the river. At last another epoch of upheaval 

 set in; the entire platform of the district was lifted several thousand 

 feet; the power of the river to corrade was restored; and with compar- 

 ative rapidity it sank the inner gorge. This becomes more than a mere 

 guess when we take account of its relation to the general category of 

 facts. Thus the great faults attest the fact that such an upheaval did 

 occur; that it occurred, too, just at the time supposed; and that in 

 amount it was quite equal and probably not more than equal to the 

 amount required. Other evidences might also be produced, but they are 

 too intricate to be discussed here.* 



We leave the Toroweap Valley and the Grand Canon, regretting that 

 all its wonderful and instructive subjects should receive such brief notice. 

 Retracing our steps up the Toroweap for a distance of about six miles, 

 we at length select one of the great lava streams on the western side. 

 Although quite steep, we may ascend it with the animals and packs 

 without serious difficulty. At the end of an arduous climb upon the 

 ragged slope, we find ourselves upon the platform of the Uinkaret. 

 Around us are the old cinder-cones, most of which are of considerable 

 dimensions. All of them have given vent to Hoods of basalt, which have 



M would, if spac7admitteilj>^ 

 inthe wall of the inner gorge directly across from VnlcanVI'hrone. Upon the very brink 

 stands the remnant of an old crater (cinder-cone) whicli has been partially under- 

 mined and destroyed by the sapping of the wall-face. A lateral gorge sets back into 

 the esplanade from the river to a distance of a mile or more. In the wall-faces are dis- 

 posed tluMlikes through which the lava came up. Their "strike" is parallel to the course 

 Of the river, and perpendicular to the course of the Toroweap fault. Two of them 

 protrude from the face of the wall aboutGOOto l,400feet below the summit ; others pro- 

 trude just at the brink. It is extraordinary that none are seen in the depths of the 

 gorge. All of the attendant circumstances are surprising and curious, and yet it has 

 frequently been noted that basalts habitually seek improbable places to erupt. 



