Page:Philosophical Transactions - Volume 145.djvu/121



Introduction,

progress of geology has demonstrated, that the portion of the crust of the globe which is accessible to us, has been formed by a series of successive operations, and that each member of the series of great changes must have required a period of vast duration for its development. We learn from the astronomer that the mean distance from our earth to the sun is ninety-five millions of miles, and that the distance which separates us from the 61st star of the Swan is 412,000 times ninety-five millions. Although he thus describes an extent of distance of which it is scarcely possible for us to form a just conception, still he expresses himself in definite terms. Not so the geologist: while the astronomer with his telescope penetrates into the remotest regions of Space, and in the known velocity of light has a scale by which he can estimate the vast distance, the geologist looks into an unfathomable abyss of Time; for no power of sounding its depth has yet been discovered. If he attempts to assign a definite term in time, for the period of the formation of any particular series of strata, even among those that belong to the most recent of the tertiary deposits, he has hitherto sought in vain for any reliable scale of measurement; and he speaks of thousands, or millions, or myriads of years or ages, just as imagination leads him to give a form to his ideas of vast immeasurable antiquity.

It is scarcely within the range of possibility that the absolute age of the earth's crust, reckoned in years backward from some historical epoch, will ever be