Jack Armstrong #9

Jack Armstrong and his friends uncover a suspicious plan involving an active volcanic crater and race against time to prevent it from being blown up. Working with authorities, they map out hideouts based on clues gathered from strange noises and church bells, eventually leading them to a crater site where they must evacuate before the volcano erupts. The adventure also features a subplot involving Betty discovering a travel folder about gemstones produced by volcanic heat, and concludes with Jack, Betty, and their companions completing their mission and enjoying time with local lumberjacks in what appears to be a remote forest area.