404 II. S. WASHINGTON ISOSTASY AND ROCK DENSITY 



For the suggestion and use of this method and for aid in the calcula- 

 tions, I am deeply indebted to Dr. Leason H. Adams, of the Geophysical 

 Laboratory. 



The isopiestic depth was calculated from different sets of data: (1), 

 the continental masses (except Antarctica, for which no reliable estimate 

 of the altitude is possible), with the Atlantic and Pacific Ocean floors; 

 (2), a set of smaller areas, 2S selected as among the most reliable for 

 density and altitude, with the ocean floors; and (3), the 11 areas within 

 the United States 29 whose densities had been calculated, the ocean floors 

 not being considered in this set. Two different assumptions were made 

 as to the average depth of the Atlantic in dealing with sets (1) and (2). 

 The first was that the depth ( — 7i m ) is the mean depth of the whole ocean 

 basin ( — 4,116), and the second was that the depth ( — h r ) is that of the 

 ridge or shelves on which the islands stand ( — 1,830). The United 

 States area was selected to represent a continental mass, uninfluenced by 

 the density of the ocean floors, because the densities and altitudes of this 

 country are the most reliable among the larger areas. The densities used 

 throughout are those calculated for water-free rock. 



Space is lacking to give here in detail all the results of these calcula- 

 tions. We shall consider, as an example, only those for set (1), the 

 continents and ocean floors calculated as water-free. The areas dealt with 

 in this set are the largest, the values for 8 and h are probably among the 

 most reliable, there is a fairly wide range in altitudes, and the question 

 of personal judgment and selection of data does not enter. 



Table V. — Densities of Continents and Ocean Floors. Water-free Basis 



Area ll 5a 5ni 5r 5a 5m 5a ■ 5r 



Meters 



North America 700 2.782 2.767 2.755 +.015 +.027 



South America 580 2.783 2.773 2.760 +.010 +.023 



Europe 300 2.790 2.786 2.776 +.004 +.014 



Africa 650 2.780 2.769 2.757 +.011 +.023 



Asia 950 2.769 2.755 2.746 +.014 +.033 



Australia 350 2.814 2.784 2.773 —.030 --.041 



Atlantic (average) — 4,116 2. 888 3.020 —.132 



Atlantic (ridge) —1,830 2.888 2.892 —.004 



Pacific (average) —4,520 3.087 3.044 3.053 —.043 —.054 



Sealevel (5 ) 2 . 801 2 . 814 



o 



Assuming Atlantic average, M m = 56.79 km. = 35.29 miles. 

 Assuming Atlantic ridge, M = 52.64 km. = 32.71 miles. 



28 Appalachia, Algonkian region, Colorado, Wyoming, Montana, California, Andes, Great 

 Britain, Madagascar, Japan, Malaysia, and New Zealand. 



20 New England, Appalachia. Algonkian region, Arkansas-Texas, Montana, Wyoming, 

 Colorado, Utah-Nevada, Arizona-New Mexico, Oregonian region, and California. 



