This is the second book in a two book set that comprise one very long story, broken into two books in order to keep the size of each book to a reasonable size.  The first book in the set was Gideon, followed immediately by this book.

Summary.  Stydle has taken control of Gideon’s Gang, and is causing great trouble on the river bank.  The gang captures several Club members, including Hawkins, and beats them severely with a whip.  Gideon returns, and reasserts control of his gang, and they are on much better behavior.  Stydle begins to form a new gang of toughs.  It is revealed that Gideon had escaped the previous winter from the School for Bad Boys in order to settle a debt his father had died with.  Gideon’s father had located a mastodon skeleton, and told Gideon where it was hidden.  Gideon assembles it and sells it for $1000, which he uses to pay off his father’s old debt, and then Gideon returns to the School for Bad Boys.  Stydle, meanwhile, knows where more valuable old bones are in a remote pit in a cave, and is trying to recover them.  He nearly dies in the attempt, an experience which reforms him and he goes away on good terms with Hawkins and the Club.  Doc Waters finds the other bones, and sells them for $5000.  Two toughs from Stydle’s old gang, seeking their final revenge, burn down the clubhouse.  Doc Waters uses the $5000 to buy a bus that he has converted into a “travelling headquarters”, and the book ends with the boys preparing to set off with Jeckerson on their next adventure in another town.