ELEPHANT’S TOOTH IN VALLEY DRIFT. 39 
expect a velocity adequate to the transportation of the large rounded boulders by which the 
elephant’s tooth is surrounded; but it must be borne in mind that Elephant Hill is scarcely 
two miles from the stupendous gateway which forms the southern entrance to the Black caiion, 
of which the walls rise in perpendicular precipices of porphyry from 800 to 1,200 feet in 
height. When the Colorado began the task of cutting down the gigantic wall at the point 
where its accumulated waters, in greater volume than now, poured down its southwestern 
declivity, the cascade which it formed must have surpassed any similar exhibition of Nature’s 
forces of which we have knowledge at the present day. The legitimate and inevitable effect 
of such a cascade would be to excavate a deep basin or channel at its foot; and subsequently, 
as the fall was diminished, to fill that excavation with boulders of gravel and sand. 
Fig. 11.—e.LEPHANT HILL FROM CAMP 55. 
I shall refer again to the interesting geological features of the vicinity of the entrance to 
Black cafion in treating of the surface geology of the country bordering the upper Colorado. 
In that connexion, the discussion of the erosions to which I have referred, as well as the indi- 
cations of a diminution of the water flowing in this stream, and the former existence of ele- 
phants in a region now too much of a desert to sustain such animals, will find an appropriate 
place. 
BLACK CANON. ° 
This magnificent cafion has been so frequently referred to in the preceding pages that but 
little more need be said of it. It is formed by the passage of the Colorado through the Black 
mountain range, the most formidable obstacle to its course that we had thus far observed. 
This chain is here about twenty miles wide, and the cafion, which cuts it diagonally, about 
twenty-five miles in length; its northern entrance being a short distance below the mouth of 
the Rio Virgen. Throughout the entire interval between its northern and southern termini 
