ISOSTASY, in Geology. — When the Great Trigonometrical Survey of India was initiated, it was found that the deflection of the plumb-line by the Himalayas was much less than the calculated amount due to the theoretical attraction of the visible mass of the mountains. Sir G. B. Airy suggested that this might be caused by the presence of a mass of matter, of less than the average density, under the mountains; this explanation was further investigated by Archdeacon J. H. Pratt, who applied the term compensation to the negative effect of the underlying defect of density, in compensating the direct effect of the attraction of the visible mass of the mountains. In 1892 Major C. E. Dutton, discussing the greater problems of physical geology, deduced a general principle that the weight of matter under any unit area of the earth's surface tended to become uniform, and suggested that this was brought about by an underground transfer of material to balance the visible surface transport from regions of erosion to those of deposition. To this principle he gave the name isostasy (ἴσος, equal, and στάσις, position), not as a synonym for Pratt's compensation, but as a name for the principle and process by which it was brought about. In 1909 there appeared a very complete and elaborate investigation of the subject, by J. F. Hayford, in which the word isostasy is used throughout as synonymous with what Pratt called compensation, and this use of the term has since become general among geodesists. Some inconvenience results from this change in the meaning attached to the word, for it is still largely understood by geologists in the sense intended by its inventor, as the process by which the fact implied by Pratt's word, compensation, is brought about (see also Geology).
See G. B. Airy, Phil. Trans., cxlv., 1855, p. 101; J. H. Pratt, Phil Trans., cxliv., 1859, p. 745; C. E. Dutton, Bull. Phil. Soc. Washington, xi., 1892, p. 51; J. F. Hayford, The Figure of the Earth and Isostasy, from Measurements in the United States (Washington 1909).
(R. D. O.)