Olbers saw in a comet's tail-- "A sudden flash and pulsation of light which vibrated for several seconds through it, and the tail appeared during the continuance of the pulsations of light to be lengthened by several degrees and then again contracted."[1] [1. "American Cyclopædia," article "Clay." 2. "Edinburgh Review," October, 1874, p. 208, 3, "Cosmos," vol. i, p. 143.] {p. 76} Now, in this perpetual motion, this conflict, these great thrills of movement, we are to find the source of the clays which cover a large part of our globe to a depth of hundreds of feet. Where are those exposures of granite on the face of the earth from which ice or water could have ground them? Granite, I repeat, comes to the surface only in limited areas. And it must be remembered that clay is the product exclusively of granite ground to powder. The clays are composed exclusively of the products of disintegrated granite. They contain but a trace of lime or magnesia or organic matters, and these can be supposed to have been infiltrated into them after their arrival on the face of the earth.[1] Other kinds of rock, ground up, form sand. Moreover, we have seen that neither glaciers nor ice-sheets now produce such clays. We shall see, as we proceed, that the legends of mankind, in describing the comet that struck the earth, represent it as party-colored; it is "speckled" in one legend; spotted like a tiger in another; sometimes it is a _white_ boar in the heavens; sometimes a _blue_ snake; sometimes it is _red_ with the blood of the millions that are to perish. Doubtless these separate formations, ground out of the granite, from the mica, hornblende, or feldspar, respectively, may, as I have said, under great laws, acted upon by magnetism or electricity, have arranged themselves in separate lines or sheets, in the tail of the comet, and hence we find that the clays of one region are of one color, while those of another are of a different hue. Again, we shall see that the legends represent the monster as "winding," undulating, writhing, twisting, fold over fold, precisely as the telescopes show us the comets do to-day. [1. "American Cyclopædia," vol. iv, p. 650.] {p. 77} The very fact that these waves of motion run through the tail of the comet, and that it is capable of expanding and contracting on an immense scale, is conclusive proof that it is composed of small, adjustable particles. The writer from whom I have already quoted, speaking of the extraordinary comet of 1843, says: