abstract:The Oka (, ) is a river in Siberia, left tributary of the Angara River. It originates in the Sayan Mountains in western Buryatia and flows through Irkutsk Oblast.
All the left bank of the river Oka is covered with adenseforest. In the 60sof the XIXth century one Russian professorKaufman took noticeof a strangetransitionalnatureinthis place.