798 
ately beneath the crust would gradually 
sink deeper and deeper, while ruptures and 
readjustments 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 dis- 
place silicon dioxide from its combinations. 
The silicates, in consequence, have been 
continuously robbed of a part, or the whole, 
of their bases. The silica thus set free goes 
ultimately to form quartz veins and quartz 
sand on or about the emerged land, while 
the bases leached out of the disintegrating 
rocks are carried out into the ocean and 
ocean-basins. A continuous disintegration 
and differentiation of materials of the 
lithosphere, accompanied by a sort of 
migration and selection among mineral 
substances, is thus always in progress. 
Through the agency of life, carbonate of 
lime accumulates in one place ; through the 
agency of winds, quartz sand is heaped up 
in another; through the agency of water, 
beds of clay, of oxides of iron and of man- 
ganese are spread out in other direc- 
tions. 
The contraction of the centrosphere sup- 
plies the force which folds and crumples the 
lithosphere. The combined effect of hydro- 
sphere, atmosphere, and biosphere on the 
lithosphere gives direction and a deter- 
minate mode of action to that force. From 
the earliest geological times the most 
resistant dust of the continents has been 
strewn along the marginal belt of the sea- 
floor skirting the land. At the present 
time the deposits over this area contain on 
the average about 70 per cent. of free and 
combined silica, mostly in the form of 
quartz sand. In the abysmal deposits far 
from land there is an average of only about 
30 per cent. of silica, and hardly any of this 
in the form of quartz sand. Lime, iron, 
and the other bases largely predominate 
in these abysmal regions. The continuous 
SCIENCE. 
[N. S. Vou. X. No. 257. 
loading on the margins of the emerged land 
by deposits tends by increased pressure to 
keep the materials of the tektosphere in 
a solid condition immediately beneath the 
loaded area. The unloading of emerged 
land tends by relief of pressure to produce 
a viscous condition of the tektosphere im- 
mediately beneath the denuded surfaces. 
Under the influence of the continuous 
shakings, tremors, and tremblings always 
taking place in the lithosphere the materials 
of the tektosphere yield to the stresses acting 
on them, and ‘the deep-seated portions of the 
terrigenous deposits are slowly carried to- 
wards, over, or underneath the emerged 
land. The rocks subsequently re-formed 
beneath continental areas out of these ter- 
rigenous materials, under great pressure 
and in hydrothermal conditions, would be 
more acid than the rocks from which they 
were originally derived, and it is well 
known that the acid silicates have a lower 
specific gravity than the intermediate or 
basic ones. By a continual repetition of 
this process the continental protuberances. 
have been gradually built up of lighter ma- 
‘terials than the other parts of the litho- 
sphere. The relatively light quartz, which 
is also the most refractory, the most stable, 
and the least fusible among rock-forming 
minerals, plays in all this the principal réle. 
The average height of the surface of the 
continents is about three miles above the 
average level of the abysmal regions. If 
now we assume the average density of the 
crust beneath the continents to be 2.5, and 
of the part beneath the abysmal regions to 
be 3, then the spheroidal surface of equal 
pressure—the tektosphere—would have a 
minimum depth of eighteen miles beneath 
the continents and fifteen miles beneath the 
oceans, or if we assume the density of the 
crust beneath the continents to be 2.5, and 
beneath the abysmal regions to be 2.8, then 
the tektosphere would be twenty-eight 
miles beneath the continents and twenty-- 
