2 /. IV. POWELL 



tive terms are needed. I shall, therefore, speak of the nucleus, 

 the rocky crust or crust, the aqueous envelope or envelope, and 

 the aerial mantle or mantle, and shall call them all spheres. For 

 the sake of clearer distinction, these spheres may be called (i) 

 the centrosphere, (2) the lithosphere, (3) the hydrosphere, and 

 (4) the atmosphere. It must be observed that the ether is com- 

 mon to all of the celestial bodies, and perhaps penetrates them 

 as it does the earth. 



The centrosphere is the chief mass and has a density of 5.6. 

 By reason of this great specific gravity, which is about twice that 

 of the rocky crust, it is often supposed to be metallic. Geologic 

 facts in a vast system lead to the induction that the centrosphere 

 does not exist in the solid state ; if it is metallic the weight 

 reduces it to a trans-solid condition. To this condition the 

 form of the earth testifies, as it is an oblate spheroid assuming 

 the figure of a fluid under the combined action of gravity and 

 rotation. There are facts which have led physicists to conclude 

 that it must have a rigidity said to be equal to that of steel. 

 This rigidity may be explained as a function of its rotation, 

 revolution, and molecular motion, when the physicist and geol- 

 ogist would be in substantial accord. 



The theory of a metallic centrosphere seems adequately to 

 account for the trans-solid state, as the metals are found to flow 

 under pressure ; but the molten material which from time to time 

 is brought to the surface from the interior of the earth never 

 reveals this metallic constitution. It may be that there is a zone 

 of matter beneath the structural rock and overlying the metallic 

 nucleus which is penetrated by heat, now here, now there, and 

 only these molten rocks are extravasated ; or it may be that the 

 solid state is limited by heat in one direction and by pressure 

 in the other in such manner that all rocks flow under great 

 pressure as do the metals. 



The stony crust has been revealed by direct penetration to a 

 depth of more than six thousand feet, but it is indirectly 

 revealed in many regions to a much greater depth, perhaps in 

 extreme cases to fifty or sixty thousand feet. 



