174 STRUCTURAL GEOLOGY. 



132) that the interior temperature of the earth increases 

 1 for every fifty-three feet in depth, and that at this rate 

 the fusing temperature of rocks would be reached at about 

 thirty miles ; and, finally, that many have thence hastily 

 concluded that the general structure of the earth is that 

 of a globe of fused rock or lava, covered with a thin shell 

 thirty miles thick. But we have also shown there the 

 untenableness of this view. There are only two other 

 views possible, and now held. Some hold that the earth 

 is truly solid throughout, excepting reservoirs of liquid 

 matter forming the foci of volcanoes. Others hold that 

 the earth consists of 1. A solid nucleus, which forms its 

 greatest part ; 2. A solid crust, comparatively thin ; and, 

 3. Separating these, a sub-crust layer of liquid or semi- 

 liquid matter, if not universal, at least over large areas. 

 There are many geological phenomena which seem to 

 make this last view most probable. 



Density of the Earth. The mean density of the 

 earth, taken as a whole, is 5.6. The density of the crust 

 is about 2.5. Therefore the density of the central parts 

 must be very much greater than 5.6. It is probably not 

 less than 15 to 16. This greater interior density is due 

 partly to a difference of material (the denser settling 

 toward the center, while the earth was still in a fused con- 

 dition), and partly to condensation by pressure. 



Crust of the Earth. The surface portion of the 

 earth diifers in many respects from the interior, and is, 

 therefore, properly called a crust : 1. It is certainly a 

 lighter portion covering a denser interior. 2. It is a cooler 

 portion, covering an incandescent interior. 3. It is, as 

 we shall see hereafter, a stratified portion covering an 

 unstratified interior. 4. It is probably an oxidized por- 

 tion covering an unoxidized or less oxidized interior (for 

 oxidation comes by contact with air and water). 5. It is 

 probably a solid shell covering a liquid or semi-liquid sub- 

 crust layer. It is this idea of a solid shell covering a 



