THE ISOPIESTIC LEVEL 



407 



analyzed and which are of less density than those which undoubtedly 

 constitute the greater part of the floor— a fact which was pointed out by 

 Iddings in his two papers cited above. Third, we have no data for the 

 average densities of such extremely high columns as those capped by the 

 Tibetan Plateau and the Pamir, which have average altitudes respectively 

 of about 4,500 and 4,000 meters. 31 



Let us now assume reasonable values for these unknown densities, ac- 

 cept the approximately known altitudes (positive or negative) for the 

 new areas, combine them with a set of the densities and altitudes of 

 known areas which will introduce into the equations a fair number and 

 distribution of intermediate data, and calculate a hypothetical value for 

 M, the isopiestic depth, on this basis. I have calculated such a hypo- 

 thetical isopiestic depth, using the following assumed data: Tibetan 

 Plateau, h === 4,500, 8 = 2.60; Pamir, h = 4,000, 8 = 2.65; Atlantic, 

 average depth, h = — 4,100, 8 = 3.00; Pacific, extreme depth, h — 

 — 5,500, 8 = 3.25. With these were reckoned the values of h and 8 for 

 Colorado, Wyoming, Montana, Appalachia, Japan, Great Britain, the 

 Atlantic ridge, and the average Pacific, as given in Table III. We are 

 thus using quite possible — indeed, quite probable — extremes and have a 

 fair number of reliable intermediate data fairly well scattered as to alti- 

 tude. Calculation by the method used in the other cases yields the values : 

 M = 58.99 kilometers and 8 = 2.830. The densities for water-free rock 

 were used, but, as we have seen above, the difference between this result 

 and that from using water-bearing rock is so small as to be negligible 

 for our purpose. 



It is not probable that the extreme values here assigned for h and for d 

 will ever be greatly changed. The altitudes can not be far wrong, and we 

 have assigned to the Tibetan and Pamir heights rock densities which are 

 about as low as can be considered possible for holocrystalline rocks. The 

 value for the material of the floors of the Pacific and other oceanic 

 "deeps" may be changed when or if we obtain some direct knowledge in 

 the future as to the exact characters of the rocks that compose them ; but 

 it is not probable that the value here assigned, which exceeds that of 

 most basaltic rocks, can be greatly increased. 



The close correspondence between the value so obtained, 59 kilometers, 

 and that of Bowie, 60 kilometers, from gravity determinations at 216 

 stations distributed over the United States, is obvious. The correspond- 

 ence may be fortuitous, but it may be permitted to express the belief that 

 it has a firmer basis than mere chance, and that it expresses the approxi- 



31 A. Heilprin : Lippincott's Gazetteer, 1906, pp. 1397 and 1832. 



