288 G. R. PUTNAM CONDITION OF THE EARTH'S CRUST 



a groundwork for extended mapping operations, two instrumental means 

 have been developed of investigating the condition of the earth's crust. 

 Anomalies in the relative direction of the plumb-line may be measured 

 by comparing astronomical positions on the earth's surface with positions 

 derived from triangulation, and anomalies in the force of gravity may be 

 determined relatively by measurements of the vibration periods of pendu- 

 lums. These anomalies evidently have a common cause, and attention 

 was first called to them by observed deflections in the plumb-line. 



Development of the Theory of Isostasy 



The study of these so-called anomalies has furnished evidence, believed 

 to be conclusive, of another fact as to the earth — that the crust, or outer 

 portion, is in a condition of approximate equilibrium, termed isostasy. 

 While the general shape of the earth is that of an oblate spheroid, the 

 actual surface departs from that shape by reason of the irregularities due 

 to the topography, such as mountain ranges, valleys, ocean basins, etcetera. 

 While these surface irregularities are small compared with the dimensions 

 of the earth, yet if the crust were of a uniform density and reached to a 

 uniform depth below sealevel, they would represent enormous inequalities 

 in the loads borne by the strata below. The materials of which the earth 

 is composed could not in general withstand such differences in pressure. 

 Under the isostatic theory, substantially equal weights are sustained on 

 equal areas at some depth below sealevel, this depth not necessarily being 

 uniform. Mountains and other elevated regions are, as it were, floated 

 by lighter materials beneath or composing them, as compared with the 

 materials beneath ocean bottoms, which regions are depressed by reason 

 of their greater density, the extent and arrangement of the differences of 

 density being sufficient to bring about an approximate state of equilib- 

 rium. 



Although there had been earlier suggestions of this nature, particu- 

 larly by Herschel in 1837 and Stokes in 1849, the idea of isostasy appears 

 to have been expressed first by Airy. In the triangulation of India the 

 fact was developed that at stations nearest to the Himalayas the plumb- 

 line was deflected toward the mountains, but to a much less extent than 

 would be required by the computed effect of the great mountain masses 

 above sealevel, the discrepancy amounting to about 10 seconds in lati- 

 tude. The explanation first proposed by Pratt was that this discrepancy 

 is due to an abnormal ellipticity of the earth for this region. Shortly 

 thereafter, in 1855, in a paper before the Eoyal Society of London, Airy 

 gave his conclusion that the apparent anomaly in India is due to the 



