Therefore, when we come to take the rate of deposit, and the vast thickness of these rocks into account, we have a basis for determining approximately the periods of the time during which these rocks have formed. Of course, it is only to those who become minutely intelligent in regard to the details that this thought will have weight. If, for instance, I should go along where a man was cutting a tree, and count the annular rings, and I had never seen a tree cut down; if he should say an hundred years ago that tree was an acorn, I should be astonished, and it would have no weight with me, for I would not have the knowledge necessary. It is necessary to become familiar with this class of phenomena. Therefore in the beginning we want to study the phenomena that are taking place on the seashore, at the mouth of the rivers, and compare these with the phenomena that we find in the rock. I recall a notable instance of this: That grand man, President Hitchcock, long since gone to rest, one of the greatest and most eminent scientists of his State, discovered tracks in the valley of the Connecticut River. He had to classify some sixty different classes of tracks, which are found in the valley of the Connecticut sandstone ledges, which, when it was soft, was admirably calculated to receive impressions. His attention was called to them, and he made them out to be bird tracks. The European naturalists were very reluctant to believe it. They did not believe that any such discovery could be made by any American. They sent over a man to see the alleged bird tracks, and he went all over the museum, and he went down to the quarry and saw all the specimens, and went away and came back again. President Hitchcock asked him about it, and he said that he did not think there were any bird tracks there. After dinner he took him out to the porch. A few days before he had found a flat stone, on which mud had been deposited, and a snipe had walked over it; the mud dried, and he thought it was an exact parallel with his bird tracks. So he put it on his porch. The gentleman said, “What have you got here?” There they were, the tracks of a bird, and a wader. He said nothing. He went away, and said they were bird tracks. He wrote afterward that he had for the first time to study how a wading bird walked on both sides of a median line. And so when he studied the habits of a bird, he saw that these fossils were bird tracks. I speak of this because it is a key.