422 THE JOURNAL OF GEOLOGY. 



The Vertical Relief of the Globe. By Hugh Robert Mill, D.Sc, 

 F.R.S.E. Scottish Geographical Magazine, April, 1890. 



The purpose of Dr. Mill's paper is to show a simple yet adequate basis 

 on which to build the superstructure of physical geography. It does not 

 attempt a discussion of the distribution and varieties of vertical relief. The 

 structure of the earth is stated most simply by describing it as an irregular 

 stony ball, covered with an ocean and an envelope of air. If the lithosphere 

 were perfectly smooth and at rest, with the hydrosphere uniformly spread over 

 its surface, the former would have the form of the terrestrial spheroid, and the 

 latter would surround it to a depth of 1.7 miles. The surface of this hypo- 

 thetical spheroid Dr. Mill calls mean sphere level. Of course, in reality the 

 surface of the lithosphere is not perfectly smooth. Parts of it are greatly 

 depressed and parts much elevated, the latter forming the land of the earth. 

 The writer proceeds to calculate the position of mean sphere level, and in 

 the absence of accurate data he uses the careful estimates of Dr. John Mur- 

 ray, which are as follows : Average depths of oceans = 2.36 miles ; average 

 height of land = .426 miles ; average thickness of hydrosphere surrounding 

 smoothed lithosphere == 1.7 mile; area of land = 55,000,000 square miles; 

 area of oceans = 141,700,000 square miles. Suppose a block of 55,000,000 

 of square miles, area and 1.7 miles deep to be cut out of the smoothed litho- 

 sphere and set down on the surface alongside the depression. No change 

 will take place in the surface of the hydrosphere. If the surface of the 141,- 

 700,000 square miles of lithosphere were reduced to uniformity, the whole 

 depressed area would lie .66 mile beneath mean sphere level, and the depth of 

 the ocean becomes 2.36 miles. To raise the land to its actual mean level 

 above the hydrosphere surface, a sufficient quantity of matter must be 

 removed from the depressed area and placed on the elevated block. Let x 

 = the thickness of the beh removed and y equal the thickness of the belt 

 when placed on the elevated block. Then x -\- y is the height of the land 

 above the actual hydrosphere level. From the data given the following equa- 

 tions are easily obtained: 



X -\- y — .426 = o 



141. 7,r - 55J/ = o 



X = .12 and y = .306 in miles. 



The average height of the land above mean sphere level is thus 1.7 + 

 .306 = 2.006 miles, and the average depth of the depressed portion beneath 

 ■ mean sphere level is .66 -j- .12 = .78 mile. 



Dr. Mill divides the earth into the three following divisions : (i) Abysmal 

 area, occupying all the depressions beneath the mean surface of the litho- 

 sphere, occupying 50 per cent, of the earth's surface; (2) Transitional area, 

 comprising all the regions above mean sphere level covered by the hydro- 

 sphere, occupying 22 per cent, of the surface; (3) Contiftental area, all the 

 lithosphere that projects above the hydrosphere, or 28 per cent, of the earth's 

 surface. J. A. B. 



