194 O. C. JONES, GEORGE D. HUBBARD Vol. XXII, No. 7 



INTRODUCTION, 



It is a characteristic of the human mind that the things 

 least easily understood are the things most desirable to know; 

 and this insatiate curiosity frequently devises means to unlock 

 the hidden and to overcome the impossible. It has been in 

 large measure this psychological impetus that has prompted 

 studies dealing with the unattainable central structure of our 

 globe, and with the slow, invisible adjustments and responses, 

 to which terrestrial matter is subject. 



By lithosphere is meant the solid or rock part of the earth, 

 not water and air, but probably all the rest from center to 

 circumference. In order to gain an accurate impression of 

 the adjustments taking place in the earth's outer part and of 

 its physical condition, it is necessary to consider the whole 

 problem of the interior of the earth. All evidence relating to 

 the physical condition of the interior of the earth or to the 

 adjustments of the earth's substance will, therefore, be first 

 drawn together without statement of its reference to the 

 problem; then conclusions will be drawn from the facts. In 

 this way the influence of theories which may or may not be 

 true will be avoided to some extent. 



Our thesis is that essentially all the problems of the dynamics 

 of the lithosphere resolve themselves into the problem of 

 adjustment of strains and stresses in the structures and 

 materials of the earth, adjustment to the equilibrium form of 

 the earth, through the force of gravity, involving diastrophism, 

 vulcanism, and gradation. 



THE ASTRONOMICAL AND PHYSICAL DATA. 



Astronomy has furnished the geologist three lines of argu- 

 ment pertaining to the physical condition of the earth's sub- 

 stance: These are precession and nutation, variation of 

 latitude, the shape of the earth. 



Precession. — Because the axis of the earth describes a 

 cone in its motions, tracing a circle about the pole of the 

 ecliptic, the equator, which preserves about the same inclination 

 to the ecliptic, moves so that its intersections with the ecliptic 

 pass in a retrograde direction opposite to the earth. Thi.<^ 

 motion is termed the precession of the equinoxes.^ Precession 



^Encyclopoedia Britannica. 



