In this Subthesis we study the solar activity of the Miocence (or Triassic) period. Measurements of the thickness of the annual tree-rings of petrified trees provide a unique method to determine the cylce of the solar activity during the life-time of the tree in pre-historic times. Study of annual tree-rings of 12 different petrified trees from the Miocene (or Triassic) period show solar cycles with a period of 6.8 - 8.4 years in contradiction to the recent 22-year-period.