446 REID— ISOSTASY AND MOUXTAIX RANGES. [April 21, 



In 1880 yir. Faye showed that the so-called "anomalies" of 

 gravity would practically disappear if, in reducing observations on 

 land to sea-level, no account were taken of the land mass above sea- 

 level ; and if, in reducing observations made on islands in mid ocean, 

 the excess of attraction of the island mass- over an equal amount 

 of sea-water were subtracted. This is equivalent to assuming that 

 the continental areas stand up on account of their low densities, but 

 that the small islands are supported by the rigidity of the crust. ^ 



In 1889 Major Button read a very remarkable paper before the 

 Philosophical Society of Washington,- in which he pointed out that 

 the mountain regions were probably continuing to rise as a result 

 of the lightening of their weight by erosional transportation and that 

 regions of deposition near the coasts were probably sinking on 

 account of the added material which they were receiving, and that 

 the forces thus brought into play would set up slow currents from 

 the regions under the sea towards the region under the mountains ; 

 and he held that the earth was not strong enough to sustain the 

 weight of great mountain ranges but that these owed their elevation 

 to the fact, as already suggested by Archdeacon Pratt, that they 

 were lighter than the material under the lowlands, or under the 

 oceans; and that there was, therefore a certain equality of weight 

 in the various segments of the earth. He gave to this theory the 

 name of isostasy, which has served to give it definiteness ever since. 

 It is to be noticed that Major Button considered the elevation of 

 mountains to be due to vertical, and not to tangential forces. 



The theory of isostasy has been much discussed by geologists 

 since ]\Iajor Button's paper; many papers have been written on the 

 subject, and the available geological evidence has been invoked in 

 support of, or against, the idea ; but it was not until very recently that 

 the real evidence, which lies in the variations of the force of gravity 

 and the deviation of the vertical, has led to definite conclusions. 



Mr. Putnam and Mr. Gilbert"'' discussed a series of gravity ob- 



^"Sur la reduction des observations du pendule an niveau de la mer," 

 C. R. de I'Acad. dcs Sciences, 1880, Vol. 90, pp. 1443-1447- 



-"Some of the Greater Problems of Physical Geology," Bull. Pliilos. 

 Soc. of Washington, 1889, Volume XL, pp. 51-64. 



^"Results of a Trans-Continental Series of Gravity Measures," Bull. 

 Philos. Soc. of IVashini^ion, 1895, Volume XIII., pp. 31-/6. 



