1890.] Geology and Paleontology. 467 
GEOLOGY AND PALEONTOLOGY. 
The Strength of the Earth’s Crust.—The term crust is 
here used to indicate the outside part of the earth, without reference 
to the question whether it differs in constitution from the interior. 
Conceive a large tank of paraffine with level surface. If a hole be 
dug in this and the material be piled in a heap at one side, the perma- 
nence of hole or heap will depend on its magnitude. Beyond a cer- 
tain limit, further investigation and heaping will be completely com- 
pensated by the flow of the material. Substitute for paraffine the 
material of the earth’s crust, and the same results will follow, but the 
limiting size of the hole or heap will be different, because the strength 
of the material is not the same. Assuming the earth to be homoge- 
neous, the greatest possible stable prominence or depression is a 
measure of the strength of its material. 
It is not believed that the earth is homogeneous, and with reference 
to the outer portion of the crust it is known that it is not composed of 
homogeneous shells. There is observational basis for the theory that 
the matter composing and lying beneath ocean beds and continents is 
lighter than the matter composing and lying beneath ocean beds, and 
many students of terrestrial physics entertain the theory that unit 
columns extending from the surface downward have everywhere the 
same weight, the height of each column being inversely as its mean 
density. In accordance with this theory, prominences and depressions 
of the surface exist in virtue of a principle of equilibrium, called 
isostatic.1 Under hydrostatic equilibrium the surface of a free liquid 
is level ; under isostatic equilibrium the surface of a non-homogeneous 
solid capable of viscous flow, is uneven. 
There are thus two possible explanations of the inequalities of 
terrestrial surface, and these may be characterized severally by the 
terms rigidity and isostasy. 
In connection with a study of Lake Bonneville, a large body of 
water temporarily filling a basin of Utah during Pleistocene time,? 
observational data were gathered bearing on the question of rigidity 
versus isostasy. 
1 For definitions of the new term ‘‘isostasy ” and its adjective " isostatic,” see Dut. 
ton in Bull. Phil. Soc., Washington, XI., p. 53, and Woodward in Am. Jour. Sci., 3d 
Series, Vol. XX XVIII, 1889, p. 351. 
?An account of Lake Bonneville may be found in the Second Annual Report of the 
U.S. Geological Survey, 1881, pp. 167-200. 
