H. Rider Haggard 
Allan and the Ice-Gods 

This novel is the final volume of the Allan Quatermain saga, and it comprises the fourth part of a loosely linked series begun with Allan and the Holy Flower. Once more Quatermain takes the hallucinogenic taduki drug, as he did in previous novels, and he finds himself reliving as Wi, an civilised man living in the barbaric ice age as part of a clan of cavemen. (Goodreads)