TRANSACTIONS OF SECTION E. 797 
on fusion, upon which rests and creeps the lithosphere. Then follow hydrosphere 
and atmosphere, with the included dzosphere.1 To the interaction of these six 
geospheres, through energy derived 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. It is admitted 
that the vast nucleus has a very high temperature, but so enormous is the pressure 
of the superincumbent crust that the melting-point of the substances in the interior 
is believed to be raised to a higher value than the temperature 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. Astro- 
nomers from a study of precession and nutation have long been convinced that the 
centrosphere must be practically solid. 
Recent seismological observations indicate the transmission of two types of 
waves through the earth—the condensational-rarefactional and the purely dis- 
tortional—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 Professors Milne and Knott point also to a fairly abrupt 
boundary or transition surface, 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-textured solids of the external crust are converted 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 so rapidly 
towards the surface as to be everywhere below the melting-point of any substance 
there under its particular pressure. 
Now, as the solid centrosphere slowly contracted from loss of heat, the primi- 
tive lithesphere, 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 
everywhere and at all times in a slight but measurable 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 plateaus, of mountain platforms, and of continents. 
From many lines of investigation it is concluded, as we have seen, that the 
centrosphere is homogeneous in structure. Direct observation, on the other hand, 
shows that the lithosphere is heterogeneous in composition. How has this hetero- 
geneity 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— 
immediately beneath the crust would gradually sink deeper and deeper, while 
ruptures and re-adjustments would become less and less frequent than in earlier 
stages. With the first fall of rain the silicates of the crust would be attacked by 
water and carbon dioxide, which can at low temperatures displace silicon dioxide 
from its combinations. The silicates, in consequence, have been continuous] 
robbed of a part, or the whole, of their bases. ‘he silica thus set free goes ulti- 
mately to form quartz veins and quartz sand on or about the emerged land, while 
1 Bios, life. 
