WHAT IS TERRA FIRMA? WILLIS. 395 



heterogeneous, if some parts were denser or lighter than others, its normal 

 figure would no longer be spheroidal. Where the lighter matter was accumu- 

 lated there would be a tendency to bulge, and where the denser matter existed 

 there would be a tendency to flatten or depress the surface. For this condition 

 of equilibrium of figure, to which gravitation tends to reduce a planetary body, 

 irrespective of whether it be homogeneous or not, I propose the name isostasy. 

 * * * We may also use the corresponding adjective, isostatic. * * * 

 The question which I propose is: " How nearly does the earth's figure approach 

 to isostasy?" ^ 



Gilbert,- in a measure, proposed an answer to Button's question. 

 He had been engaged in original studies of the rigidity or strength 

 of the earth's crust and had calculated that there was a limit to the 

 mass which it could support without yielding. He expressed his 

 view very conservatively, saying: 



It is believed that the following theorem or working hypothesis is worthy 

 of consideration and of comparison with additional facts : Mountains, moun- 

 tain ranges, and valleys of magnitude equivalent to mountains, exist generally 

 in virtue of the rigidity of the earth's crust; continents, continental plateaus, 

 and oceanic basins exist in virtue of isostatic equilibrium in a crust hetero- 

 geneous as to density. 



Researches as to the distribution of lighter and denser masses in 

 the outer earth have been gi'eatly extended and highly refined since 

 1889. Button's general law is recognized as true. The larger ele- 

 vations and hollows of the earth's surface are due to the balance of 

 lighter a'<nd denser masses. Gilbert's suggestion that mountain-like 

 masses and hollows are rigidly supported, commands consideration 

 by conservative students. It is, however, apparently contradicted by 

 the exhaustive calculations of the geodesist, Hayford, who concludes 

 that the balance postulated by Button extends to masses which are 

 much smaller than any which Button or Gilbert regarded as prob- 

 ably in equilibrium. In order to understand the" present state of the 

 problem we may brieflj^ review the methods that have been employed 

 in making observations. 



Gravity is the force which causes bodies to fall toward the earth or 

 a pendulum to swing. Its intensity may be measured by the velocity 

 attained by a falling body at the end of a second, or by the number 

 of swings that a pendulum of definite length will make in a definite 

 time. The latter method of measurement is capable of very great 

 accuracy and is used for all observations of the intensity of gravity 

 on land. In order that the determinations may attain the desired 

 precision and yet be carried out within a reasonable time, a highly 

 specialized apparatus is used. The form employed by the Coast 



1 Button, C. E. On some of the Greater Problems of Physical Geology. Phil. Soc. 

 Wash., Bull., vol. 11, pp. .51-64, 1889. 



2 Gilbert, G. R. The Strength of the Earth's Crust (abstract). Geol. Soc. Am., Bull, 

 vol. 1, pp. 23-25, 1889. 



