We
only know the mass of the whole Earth and its mean density by comparing it
with the open strata, which alone are accessible to us. In the interior of
the Earth, where all knowledge of its chemical and mineralogical character
fails, we are again limited to as pure conjecture, as in the remotest bodies
that revolve round the Sun. We can determine nothing with certainty
regarding the depth at which the geological strata must be supposed to be in
state of softening or of liquid fusion, of the cavities occupied by elastic
vapor, of the condition of fluids when heated under an enormous pressure, or
of the law of the increase
p 161
of density from the upper surface to the center of the Earth.
The consideration of the increase of heat with the increase of depth toward
the interior of our planet, and of the reaction of the interior on the
external crust, leads us to the long series of volcanic phenomena. These
elastic forces are manifested in earthquakes, eruptions of gas, hot wells,
mud volcanoes and lava currents from craters of eruption and even in
producing alterations in the level of the sea.*
[footnote] * [See Daubeney 'On Volcanoes', 2d edit., 3848, p. 539, etc., on
the so called 'mud volcanoes', and the reasons advanced in favor of adopting
the term "salses" to designate these phenomena.
Pages:
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335