A cavern at the top of a cliff :
The universe, matter
A stream of water flowing through:
Elements formed by matter
Marble beams where Naiads weave purple garments :
Souls descending into generation and acquiring body.
Related to human nature of incarnated souls
Marble Beams Bones Purple Garments Flesh Bowls and urns filled with honey provided by bees :
Earth symbols - Nutritive, generative and preservative powers of nature.
A North gate for mankind :
Tropic of Cancer -Summer, Moon, North gate where the souls descend.
South gate for immortals :
Tropic of Capricorn - Winter, Saturn, South gate of ascent.
An olive tree :
Related to Athena, Divine wisdom. Peace, Victory. The Intellectual Principle that governs nature.
Basically, theses images represent the process of the soul's descending incarnation from it's cosmic origin and it's experience in relation to natural process up to the point of disembodiment and return to it's cosmic origin.
In general, Porphyry explains these symbols in relation to traditional Greek religious symbols and other mystical traditions such as Mithraism and Orphism. The symbols are all implicitely related to the lesser mysteries in ancient traditions such the mysteries of Eleusis and Attis, related to processes of generation and fertility.