“4. AND AFTER GREAT THROES SHE (THE EARTH) CAST OFF HER OLD THREE AND PUT ON HER NEW SEVEN SKINS, AND STOOD IN HER FIRST ONE.
The Earth is said to cast off her old three skins, because this refers to the three preceding Rounds she has already passed through, the present being the fourth Round out of the seven.
At the beginning of every new ROUND, after a period of “obscuration”, the Earth (as do also the other six “earths”) casts off, or is supposed to cast off, her old skins as the serpent does.
The “Seven Skins”, in the first of which she now stands, refer to the seven geological changes which accompany and correspond to the evolution of the Seven Root-Races of Humanity.”
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 