Crosby.] 446 [November 7, 



matter is attended by farther oxidation and the evolution of con- 

 siderable heat. So that it is conceivable that a higher tempera- 

 ture may reign in the lower portion of the hydrated crust than 

 in the regions immediately below the limit reached by the water ; 

 and it will not escape observation that this higher temperature is 

 developed at the point where the other conditions are most favor- 

 able to the formation of a plastic stratum. If we hold that in the 

 anhydrous globe the pressure is everywhere more than a match 

 for the temperature, it is yet difficult to believe that it can be so 

 in the lower levels of the hydrated crust, where moisture cooper- 

 ates with a temperature of from 2,000° to possibly 5,000° Fah. In 

 other words, from these generally accepted principles the infer- 

 ence flows almost irresistibly that the earth possesses a plastic 

 zone, and that this is near the surface ; and, as already pointed 

 out, this is in harmony with the conclusions of Thomson and 

 Darwin, if we make the not unreasonable assumption that the 

 central portions of the earth possess a degree of rigidity greater 

 than the mean which they require. 



One fact to which considerable weight has been attached by 

 Le Conte * as an argument against a plastic zone is rapidly disap- 

 pearing. I refer to the elliptical form of the earth's equatorial 

 section. It is certainly true that the existence of such a zone 

 would be extremely improbable, if the inequality of the equatorial 

 radii was very marked. Now the values of these radii usually 

 accepted are those determined by Col. A. R. Clark, of the British 

 Ordnance Survey, according to whom the major exceeds the mi- 

 nor radius by 6,378 feet, the major axis intersecting the meridian 

 of 15° 34' E. But Colonel Clark has published a revision of his 

 results, founded on new data ; and he now finds the difference 

 between the equatorial radii to be only 1524 feet; whilst the me- 

 ridian of the major axis is 8° 15' W. ; and the polar radius is 

 lengthened 1000 feet. Colonel Clark, himself, evidently regards 

 the ellipsoidal form of the equator as doubtful. Thus, there is, 

 at all events, no proved result of geodesy opposed to the hypoth- 

 esis of a plastic zone in the earth. 



On the other hand, there are many important and generally 

 admitted geological facts which can be satisfactorily explained 

 only on the supposition of a yielding stratum at a moderate dis- 

 1 Amer. Jour. Sci. (3), iv, p. 351. 



