Ragnarok The Age of Fire and Gravel Author:Ignatius Donnelly 1887. Partial Contents: Characteristics of the drift; Was it caused by an iceberg or continental ice sheet? Did a comet strike the earth? Legends; Nature of myths; Did man exist before the drift? Cave life; Age of darkness; Triumph of the sun; Fall of the clay and gravel; Was preglacial man civilized? Scene of man's survival; The Bridge.