Changes in the Position of the Axis of the Earth's Crust. 543 



It will of course be borne in mind that the elevations and depres- 

 sions of the surface of the globe are not, on the theory now under 

 consideration, regarded according to the proportion they bear to the 

 earth's radius, but according to their relation to the thickness of the 

 earth's crust ; and that, even assuming Mr. Hopkins's extreme esti- 

 mate to be true, yet elevations or depressions, such as we know to 

 have taken place, of 8000 or 10,000 feet, bear an appreciable ratio 

 to the 800 or 1000 miles which he assigns as the thickness of the 

 earth's crust. 



It is, however, to be remarked that the extremely ingenious specu- 

 lations of Mr. Hopkins are based on the phenomena of precession 

 and nutation, and that if once the possibility of a change in the 

 position of the axis of rotation of the earth's crust be admitted, it 

 is not improbable that the value of some of the data upon which 

 the calculations of these movements are founded may be affected. 



The supposition of the thickness of the crust being so great seems 

 also not only entirely at variance with observed facts as to the in- 

 crease of heat on descending beneath the surface of the earth, but 

 to have been felt by Mr. Hopkins himself to offer such obstacles to 

 any communication between the surface of the globe and its interior, 

 that he has had recourse to an hypothesis of large spaces in the crust 

 at no great depth from the surface, and filled with easily- fusible ma- 

 terials, in order to account for volcanic and other phenomena. 



But though it may be possible to account for volcanoes upon 

 such an assumption, yet, as already observed, the phenomena of 

 elevation and depression, such as we find to have taken place, and 

 more especially the existence of vast geological faults, cannot without 

 enormous difficulty be reconciled with such a theory. 



Taking the increment of heat as 1° Fahrenheit for every 55 or 60 

 feet* in descent, a temperature of 2400° Fahr. would be reached at 

 about 25 miles, sufficient to keep in fusion such rocks as basalt, green- 

 stone, and porphyry ; and such a thickness appears much more con- 

 sistent with the fluctuations in level,, and the internal contortions and 

 fractures of the crust which are everywhere to be observed. Sir 

 William Armstrong, on the assumption of the temperature of sub- 

 terranean fusion being 3000° Fahr., considers that the thickness of 

 the film which separates us from the fiery ocean beneath would be 

 about 34 miles. 



Even assuming a thickness of 50 miles, so as to make still greater 

 allowance for the increased difficulty of fusion under heavy pressure, 

 the thickness of the crust would only form one-eightieth part of the 

 radius of the earth ; or if we represent the earth by a globe 1 3 

 feet in diameter, the crust would be one inch in thickness, while the 

 difference between the polar and equatorial diameters would be half 

 an inch. 



In such a case, the elevation or wearing away of continents such 

 as are at present in existence, rising, as some of them do, nearly a 

 quarter of a mile on an average above the mean sea-level, would 

 cause a great disturbance in the equilibrium of the crust, sufficient 



• * Page, ' Advanced Text-book of Geology,' p. 30. ' 



