236 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Crandall volcano, which, if reckoned from the level of the ancient surface 

 of the limestone in the neighborhood of the gabbro core, are 13,000, 13,400, 

 and 13,800 feet. These results are sufficiently close to one another when 

 it is noticed that the peaks used as datum points lie in an arc of 133° and 

 at distances of from 9 to 14 miles from the center — from Index Peak in the 

 north to Pollux Peak in the south. Moreover, the highest altitude is 

 obtained by using Pollux Peak as a base of reckoning, and this mountain 

 lies within the radius of other volcanic centers near the head of Stinking- 

 water River, and has probably been built up by lavas from two great 

 centers. 



A volcano 13,400 feet in elevation with a radius of 20 miles is not so 

 large as many in existence at the present day. The volcanoes of Hawaii 

 are familiar examples. Of these, Mauna Loa, with an altitude of 13,675 

 feet, has a maximum radius of about 40 miles and a minimum radius of 20. 

 Mauna Kea, 13,805 feet in height, has an average radius of 20 miles. These 

 estimates, it should be remembered, are taken from the sea line. The 

 heights of these volcanic piles above their actual base, and the real diameters 

 of their cones, are not known. The profile of Kea is shown on the same 

 scale as that of Etna and has nearly the same outline (PI. XXXII). A 

 volcano with the profile of Vesuvius, if as large in diameter as Etna, would 

 be 5,800 feet higher, or about 16,600 feet in height. 



The conclusion, therefore, that the ancient volcano of Crandall Basin 

 rose to an elevation of 13,400 feet above the preexisting surface of the 

 limestone is Avithin reasonable limits, and is probably too low. Upon this 

 basis we find that the gabbro core in Hurricane Mesa must have solidified 

 at a distance of 10,000 to 12,000 feet below the level of the ancient crater. 

 Erosion must have removed 10,000 feet from the highest portion of the 

 volcano to the level of the mountain tops, and 4,000 feet more into the val- 

 leys between them, thus cutting 14,000 feet vertically below what was 

 once the summit of the volcano. At Index Peak the present topography 

 shows an erosion of nearly 5,000 feet from the summit of the peak to the 

 valley of Clark Fork. 



The foregoing estimates were based on the assumption that the tops 

 of the highest mountains of horizontal lava flows had not been materially 

 affected by erosion; hence we must regard the calculated amount of erosion 

 as a minimum. There seems to be no way of avoiding this conclusion, 



