DECEMBER 1, 1899.] 
of these six geospheres, through energy de- 
rived from internal and external sources, 
may be referred all the existing superficial 
phenomena of the planet. 
The vast interior of the planetary mass, 
although not under direct observation, is 
known, from the results of the astronomer 
and physicist, to have a mean density of 
5.6, or twice that of ordinary surface rock. 
The substances brought within the reach of 
observation in veinstones, in lavas, and 
hypogene rocks—by the action of water as 
a solvent and sublimant—warrant the belief 
that the centrosphere is largely made up of 
metals and metalloids with imprisoned 
gases. Itis admitted that the vast nucleus 
has a very high temperature, but so enor- 
mous is the pressure of the super-incum- 
bent crust that the melting-point of the 
substances in the interior is believed to be 
raised to a higher value than the tem- 
perature there existing—the centrosphere 
in consequence remains solid, for it may be 
assumed that the melting-point of rock- 
forming materials is raised by increase of 
pressure. Astronomers from a study of 
precession and nutation have long been 
convinced that the centrosphere must be 
practically solid. 
Recent seismological observations indi- 
cate the transmission of two types of waves 
through the earth—the condensational-rare- 
factional and the purely distortional—and 
the study of these tremors supports the 
view that the centrosphere is not only solid, 
but possesses great uniformity of structure. 
The seismological investigations of Pro- 
fessors Milne and Knott point also to a 
fairly abrupt boundary or transition sur- 
face, where the solid nucleus passes into 
the somewhat plastic magma on which the 
firm upper crust rests. 
In this plastic layer or shell—named the 
tektosphere—the materials are most probably 
in a state of unstable equilibrium and 
bordering on fusion. Here the loose-tex- 
SCIENCE. 
797 
tured solids of the external crust are con- 
verted into the denser solids of the nucleus 
or into molten masses, at a critical point of 
temperature and pressure; deep-seated 
rocks may in consequence escape through 
fissures in the lithosphere. Within the 
lithosphere itself the temperature falls off 
s0 rapidly towards the surface as to be 
everywhere below the melting-point of any 
substance there under its particular pres- 
sure. 
Now, as the solid centrosphere is slowly 
contracted from loss of heat, the primi- 
tive lithosphere, in accommodating itself— 
through changes in the tektosphere—to 
the shrinking nucleus, would be buckled, 
warped, and thrown into ridges. That these 
movements are still going on is shown 
by the fact that the lithosphere is every- 
where and at all times in a slight but meas- 
ureable state of pulsation. The rigidity of 
the primitive rocky crust would permit of 
considerable deformations of the kind here 
indicated. Indeed, the compression of 
mountain chains has most probably been 
brought about in this manner, but the same 
cannot be said of the elevation of plat- 
eaus, of mountain platforms, and of con- 
tinents. 
From many lines of investigation it is 
concluded, as we have seen, that the centro- 
sphere is homogeneous in structure. Direct 
observation, on the other hand, shows that 
the lithosphere is heterogeneous in compo- 
sition. How has this heterogeneity been 
brought about? The original crust was 
almost certainly composed of complex and 
stable silicates, all the silicon dioxide being 
in combination with bases. Lord Kelvin 
has pointed out that, when the solid crust 
began to form, it would rapidly cool over 
its whole surface ; the precipitation of water 
would accelerate this process, and there 
would soon be an approximation to present 
conditions. As time went on the plastic or 
critical layer—the tektosphere—immedi- 
