STAIRCASE FARMS OF THE ANCIENTS _By O. F. Cook_
National Geographic Magazine, Volume 29, No. 5
FURTHER EXPLORATIONS IN THE LAND OF THE INCAS _By Dr. Hiram Bingham_
National Geographic Magazine, Volume 29, No. 5
THE STORY OF MACHU PICCHU _By Dr. Hiram Bingham_
National Geographic Magazine, Volume 27, No. 2