miles in diameter. The volcano is now eroded down to the frozen magma in its vent that makes up the Kisingiri hills on the mainland opposite Rusinga, and the surrounding remnants of the cone: the semicircular Rangwa mountain range, and the islands of Rusinga and neighboring Mfangano Island. This rift valley volcano on the southern flank of the now-inactive Winam Gulf tapped much deeper in the mantle than oceanic or subduction zone volcanos, and its lavas and explosive ash clouds thus contained much more carbonate and alkali than normal. This meant that even though the Miocene environment was a tropical rainforest, the chemistry of the successive ash beds was that of a desert dry lake, preserving everything from caterpillars and berries to apes and elephants in an unusual situation found only in a few other East African ex-volcanos, notably Menengai and Homa Mountain in western Kenya, Napak and Mount Elgon in Uganda, and the much younger Ol Doinyo Lengai in Tanzania, which created the fossil beds of Olduvai Gorge
