The accretion hypothesis.  On the other hand, it has been recently suggested that the earth may have grown to its present size by the gradual accretion of meteoritic masses.  Such cold, stony bodies might have come together at so slow a rate that the heat caused by their impact would not raise sensibly the temperature of the growing planet.  Thus the surface of the earth may never have been hot and luminous; but as the loose aggregation of stony masses grew larger and was more and more compressed by its own gravitation, the heat thus generated raised the interior to high temperatures, while from time to time molten rock was intruded among the loose, cold meteoritic masses of the crust and outpoured upon the surface.

It is supposed that the meteorites of which the earth was built brought to it, as meteorites do now, various gases shut up within their pores.  As the heat of the interior increased, these gases transpired to the surface and formed the primitive atmosphere and hydrosphere.  The atmosphere has therefore grown slowly from the smallest beginnings.  Gases emitted from the interior in volcanic eruptions and in other ways have ever added to it, and are adding to it now.  On the other hand, the atmosphere has constantly suffered loss, as it has been robbed of oxygen by the oxidation of rocks in weathering, and of carbon dioxide in the making of limestones and carbonaceous deposits.

While all hypotheses of the earth’s beginnings are as yet unproved speculations, they serve to bring to mind one of the chief lessons which geology has to teach,—­that the duration of the earth in time, like the extension of the universe in space, is vastly beyond the power of the human mind to realize.  Behind the history recorded in the rocks, which stretches back for many million years, lies the long unrecorded history of the beginnings of the planet; and still farther in the abysses of the past are dimly seen the cycles of the evolution of the solar system and of the nebula which gave it birth.

We pass now from the dim realm of speculation to the earliest era of the recorded history of the earth, where some certain facts may be observed and some sure inferences from them may be drawn.

The Archean.

The oldest known sedimentary strata, wherever they are exposed by uplift and erosion, are found to be involved with a mass of crystalline rocks which possesses the same characteristics in all parts of the world.  It consists of foliated rocks, gneisses, and schists of various kinds, which have been cut with dikes and other intrusions of molten rock, and have been broken, crumpled, and crushed, and left in interlocking masses so confused that their true arrangement can usually be made out only with the greatest difficulty if at all.  The condition of this body of crystalline rocks is due to the fact that they have suffered not only from the faultings, foldings, and igneous intrusions of their time, but necessarily, also, from those of all later geological ages.