1883.] 445 ' Crosby.] 



Dana has correctly stated, 1 the rigidity of slowly solidified rock, 

 at the earth's surface, is beyond that of glass or steel. 



If we may assume that rigidity varies with varying tempera- 

 ture and pressure according to some simple and similar, though 

 contrary, laws, then by the side of Thomson's temperature curve 

 we may construct a curve for pressures, based on any probable 

 law of the downward increase of specific gravity ; and readily 

 observe where the relations of temperature and pressure are most 

 favorable for a plastic zone ; *'. e., where the pressure is least rel- 

 atively to the temperature. This region must be comparatively 

 superficial, since the temperature increases downward rapidly at 

 first and very slowly at greater depths ; and I have found in this 

 way that the ratio of temperature to pressure probably reaches 

 its maximum between 40 and 80 miles below the earth's surface. 



But the experiments of Daubree have shown, contrary to the 

 old idea, that the capillary absorption of water by the earth's 

 crust, within certain limits at least, is facilitated, rather than 

 hindered, by the high subterranean temperatures ; and hence we 

 are warranted in believing that the crust is hydrated to a very 

 great depth. Now the principle is generally accepted that water 

 reduces in a marked degree the fusing points of rocks, liquefac- 

 tion resulting from aqueo-igneous fusion at temperatures far below 

 those of purely igneous softening. Therefore, since the minimum 

 ratio of pressure to temperature occurs so near the surface, water 

 becomes an important element in locating the most probable po- 

 sition of a plastic zone. The maximum plasticity, other things 

 being equal, will be determined, approximately at least, by the 

 minimum ratio of the pressure to the product of moisture and 

 temperature ; and this probably exists nearer 40 miles than 80 

 miles below the surface. Several well established and entirely 

 independent facts — such as the high specific gravity of the earth 

 as a whole, the excess of basic elements in certain eruptive rocks, 

 and the metallic nature of many meteorites — point unequivo- 

 cally to the conclusion that the earth's interior consists chiefly or 

 entirely of metals, among which iron must occupy a prominent 

 position. Now it may very well be that, as Judd has suggested 

 in his recent work on volcanoes, the access of water to the outer 

 portions of this highly heated but only partially oxidized earth- 

 1 Amer. Jour. Sci. (3), vi, p. 11. 



