The Plan of the Earth and its Causes. — Gregory. 145 
Sphere without Polar flattening 
Stromboli _ Ve^uv;u s ^Slanc 
Ueaiterrancan flocr 
Depth of Atlanli- 
Lisbon earthquahc 
1 
Depth of too th. earth's radius 
riG. 7. DIAGRAM OF RELATIVE EXTENT OP 
INEQUALITIES OX THE EARTH's SUR- 
FACE, a, A TRUE SCALE CURVE OF PART 
OF earth's surface; b, sector of 
CIRCLE, SHOWING RELATIVE SIZE OF 
zone included WITHIN fl TO THAT OF 
THE EARTH. 
This diagram also serves to show tliat the amount of con- 
traction in the earth necessary to allow tetrahedral deforma- 
tion is very small. This is important because, as Lord Kelvin 
has shown, the amount of contraction allowable during the 
later stages of the earth's history is very limited. But geol- 
ogists have the authority of Prof. Darwin for accepting a cer- 
tain amount of contraction. "A cooling celestial orb must 
contract by a perceptible fraction of its radius after it has con- 
solidated,"' he tells us, and his considerations "only negative the 
hypothesis of any large contraction of the earth since the 
moon has existed.'"'' And, unlike the contraction theory of 
the origin of mountain chains, the theory of the tetrahedral 
deformation of the lithosphere requires only a small amount 
of radial contraction. 
Finally^ it may be urged that even such deformation as 
the tetrahedral theory requires is impossible, since physicists 
have taught us that the earth is rigid. To this objection it is 
only necessary to reply that Lord Kelvin's rigidit^' argu- 
• Phil. Trans., vol. 170, pp: 522, 523. 
