Granted:  but remember that the clay did not come to the earth as clay, but as a finely comminuted powder or dust; it packed into clay after having been mixed with water.  The particles of this dust must have been widely separated while in the comet’s tail; if they had not been, instead of a deposit of a few hundred feet, we should have had one of hundreds of miles in thickness.  We have seen, (page 94, ante,) that the tail of one comet was thirteen million miles broad; if the particles of dust composing that tail had been as minute as those of clay-dust, and if they had been separated from each other by many feet in distance, they would still have left a deposit on the face of any object passing through them much greater than the Drift.  To illustrate my meaning:  you ride on a summer day a hundred miles in a railroad-car, seated by an open

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window.  There is no dust perceptible, at least not enough to obscure the landscape; yet at the end of the journey you find yourself covered with a very evident coating of dust.  Now, suppose that, instead of traveling one hundred miles, your ride had been prolonged a million miles, or thirteen million miles; and, instead of the atmosphere being perfectly clear, you had moved through a cloud of dust, not dense enough to intercept the light of the stars, and yet dense enough to reflect the light of the sun, even as a smoke-wreath reflects it, and you can readily see that, long before you reached the end of your journey, you would be buried alive under hundreds of feet of dust.  To creatures like ourselves, measuring our stature by feet and inches, a Drift-deposit three hundred feet thick is an immense affair, even as a deposit a foot thick would be to an ant; but, measured on an astronomical scale, with the foot-rule of the heavens, and the Drift is no more than a thin coating of dust, such as accumulates on a traveler’s coat.  Even estimating it upon the scale of our planet, it is a mere wrapping of tissue-paper thickness.  In short, it must be remembered that we are an infinitely insignificant breed of little creatures, to whom a cosmical dust-shower is a cataclysm.

And that which is true of the clay-dust is true of the gravel.  At a million miles’ distance it, too, is dust; it runs in lines or streaks, widely separated; and the light shines between its particles as it does through the leaves of the trees

     “And glimmering through the groaning trees
     Kirk Alloway seems in a blaze;
     Through every bore the beams are glancing.”

But another says: 

“Why do you think the finer parts of the material of the comet are carried farthest back from the head?”

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Because the attractive power lodged in the nucleus acts with most force on the largest masses; even as the rock is not so likely to leave the earth in a wind-storm as the dust; and in the flight of the comet through space, at the rate of three hundred and sixty-six miles per second, its lighter substances would naturally trail farthest behind it; for—­