Crosby.] 444 [November 7, 



has already supplanted, in the minds of the great majority of 

 geologists, the view but recently almost universally held that be- 

 low a comparatively thin solid crust the globe is yet wholly 

 liquid. 



While the reconstruction of dynamical geology on the basis of 

 a solid earth undoubtedly marks a great advance in the science, 

 yet it appears to me that some eminent writers have carried the 

 new views too far, and made certain portions of the earth too 

 solid. Thomson asks for an average rigidity equal to that of 

 steel, and Darwin says that no considerable portion of the inte- 

 rior of the earth can even distantly approach the fluid state ; but 

 this is not necessarily inconsistent, as many appear to think, with 

 the existence of a shallow, discontinuous, plastic zone between 

 the rigid crust and a nucleus more rigid than steel. 



One of the most important results reached by Thomson is 

 that up to this time the refrigeration of the earth has been lim- 

 ited to the superficial portions, that below a depth not exceeding 

 300 miles the temperature ceases to increase downward at a sen- 

 sible rate, and that the maximum terrestrial temperature is prob- 

 ably not above 7,000° to 10,000° Fah. In other words, the great 

 interior portion of the earth, embracing five-sixths of its volume 

 and probably seven-eighths of its mass, has a sensibly uniform 

 temperature, which is not incomparably higher than temperatures 

 with which we are familiar on the earth's surface. But the pres- 

 sure to which materials in the interior of the earth are subjected 

 increases steadily all the way to the centre. Therefore, since 

 heat acts against, while pressure promotes, rigidity, if the earth- 

 matter, as is generally believed, is solid at a depth of 300 miles in 

 obedience to pressure (about 2,000,000 pounds per square inch) 

 and in spite of its high temperature, it is to my mind quite con- 

 ceivable that at greater depths, where the temperature is not 

 sensibly, or at most not greatly, higher, a pressure ten times 

 greater (or 20,000,000 pounds per square inch) may induce a de- 

 gree of rigidity exceeding that of steel or any substance with 

 which we are acquainted. 



The extreme probability, if not the absolute certainty, of the 

 existence of this high degree of rigidity in the central portions of 

 the earth becomes apparent when we consider that, as Professor 



