And this is how such rain-drops immortalize themselves; this is the interpretation of their handwriting on the walls. Along the dry shore of an ancient sea when the tide was out, rain-drops fell on the sand and dust. Tides often come in with a rush, in wild waves driven by the wind, but when there is no wind and no waves rolling in from far distant storms the tide may overspread such delicate things as the imprint of rain-drops with a thin protecting film of mud. This was what happened to our little rain pits. Later tides overlaid them deeper from day to day, and in course of time both the layer containing the rain-drop prints and the overlying layers of sediment turned to stone. Often the heat of a summer sun will bake these rain-drop designs and this you see helps; it holds the impression until the tide can come in and spread its protecting film. Many imprints of rain-drops and of the feet of reptiles are found in the sandstone underlying the coal seams in eastern Pennsylvania, and they are always, I am told, covered with a fine powdery material, which was once the slime and mud of the tide. Such rain marks are often found also in slate. Wouldn't you like to have a slate with one of these rain-drop autographs on it? [Illustration: RAIN-DROP AUTOGRAPHS OLDER THAN THE PHARAOHS] Here, by the way, is a very important thing these rain-drops tell. Says Professor Shaler: "They tell us that the ordinary machinery of the atmosphere was operating in those days very much as it is to-day, and that the climate was much the same."[55] [Footnote 55: This quotation is from Doctor Shaler's "Nature and Man in America," a book you should read, as you should all of Doctor Shaler's books. No one has observed so many interesting things in the field of geology and few have written about them so simply or reasoned about them so well.] So, he argues, the great Ice Age couldn't have been due to change of climate, but to the other things that we read about in Chapter II. For they even know in what ages different records of rain-drops were made because they are found in rocks laid down in different periods; and one of the periods in which they are found was that in which the North Pole ice and its neighbors came down and made us those long visits. STORY OF A STROLL IN THE RAIN Another story found in museums is written in slate--not by a rain-drop but by a living creature. The slate shows the track of a reptile with feet like a bird. Evidently he was strolling along in the rain; for there you see the marks of the rain-drops right among the marks of his feet, and in the footprints themselves. Being a reptile who spent much of his time in or near the water he no doubt enjoyed these little pats of the rain-drops as he went along. BUT THIS STROLL WAS TAKEN IN THE SUN