"See how the water has plowed through the ground," said Frank, pointing to a gully the rain had made in the path. "It took a good many rains to make that gully," said Donald. "There was a little creek here for a while," said Frank. "The water has all run off now, but it has spoiled the path." "Will the gully get deeper every time it rains?" asked Susie. "Of course," said Donald. "That's what makes it." "Why does the water run along the path?" asked Uncle Robert. "Because it is lower than the ground on each side," said Frank. "How deep do you think the water will dig into the path if we do not fill it up?" asked Uncle Robert. "Oh, way, way down. I suppose," said Donald. "But if grass grew on the path," said Frank, "the water wouldn't wear the ground away. We will have to fill it up with stones." "See these pebbles, uncle," said Susie. "How did they get here? They look just like those we saw on the island." "Do you remember what I told you about the bowlders on the island?" "Yes, you said the bowlders were made by ice," answered Susie. "Did the ice make these pebbles?" "Perhaps so, and perhaps the river made them and left them here." "What! that river away down there? How could it get up here?" "That river away down there once flowed right over this ground," said Uncle Robert. "This slope," pointing just above, "was its bank, and the ground under our feet its bed." "That must have been a hundred years ago," said Donald. "Yes, a great many hundred years ago. You see the work this bit of a stream has done in the path? Many rivers begin just this way. They are cutting and changing the earth all the time." They had now come to the spring nearly at the foot of the slope. On sultry summer days it was a cool, inviting spot. The low-spreading branches of a beautiful bur oak shaded the little stream where it gushed from the outcropping limestone. "Do you want a drink?" asked Susie, taking the tin dipper which always hung by the spring. "Thank you, dear. How cool it is! It makes me think of the old spring in the hayfield where I used to work when I was a boy." "The rain has not made the spring run any faster," said Donald. "Where does this water come from?" asked Uncle Robert. "From out of the ground," said Susie. "How does it get into the ground?" [Illustration: The spring.] "It's always there, isn't it?" said Susie. "The spring runs all the time. I fill my pail here every day in the summer." "Yes, don't you remember when the wells all dried up last summer," said Frank, "that the spring was all right?" "Well, then, where has the water gone that fell to-day?" asked Uncle Robert. "Most of it has run off into the creek and river," said Donald. "It would look just like a lake if it was an inch and a half deep all over the ground." "Some of it has soaked into the ground," said Frank. "How deep down into the ground?" asked Uncle Robert. "Down to China," laughed Donald. "How deep do you have to dig to find water--to China?"