This _débris_ arranges itself in a regular order: the largest fragments are on or nearest the head; the smaller are farther away, diminishing in regular gradation, until the farthest extremity, the tail, consists of sand, dust, and gases. There is a continual movement of the particles of the tail, operated upon by the attraction and repulsion of the sun. The fragments collide and crash against each other; by a natural law each stone places itself so that its longest diameter coincides with the direction of the motion of the comet; hence, as they scrape against each other they mark each other with lines or _striæ_, lengthwise of their longest diameter. The fine dust ground out by these perpetual collisions does not go off into space, or pack around the stones, but, still governed by the attraction of the head, it falls to the rear and takes its place, like the small men of a regiment, in the farther part of the tail. Now, all this agrees with what science tells us of the constitution of clay. {p. 73} "It is a finely levigated silico-aluminous earth--formed by the disintegration of feldspathic or granite rocks."[1] The particles ground out of feldspar are finer than those derived from mica and hornblende, and we can readily understand how the great forces of gravity, acting upon the dust of the comet's tail, might separate one from the other; or how magnetic waves passing through the comet might arrange all the particles containing iron by themselves, and thus produce that marvelous separation of the constituents of the granite which we have found to exist in the Drift clays. If the destroyed world possessed no sedimentary rocks, then the entire material of the comet would consist of granitic stones and dust such as constitutes clays. The stones are reduced to a small size by the constant attrition: "The stones of the 'till' are not of the largest; indeed, bowlders above four feet in diameter are comparatively seldom met with in the till."[2] And this theory is corroborated by the fact that the eminent German geologist, Dr. Hahn, has recently discovered an entire series of organic remains in meteoric stones, of the class called _chrondites_, and which he identifies as belonging to classes of sponges, corals, and crinoids. Dr. Weinland, another distinguished German, corroborates these discoveries; and he has also found fragments in these stones very much like the youngest marine chalk in the Gulf of Mexico; and he thinks he sees, under the microscope, traces of vegetable growth. Francis Birgham says: [1. "American Cyclopædia," article "Clay." 2. "The Great Ice Age," p. 10.] {p. 74}