Many of the problems of oil shale are similar to those of the metal mining industry. The first similarity is that of sacrificing quality for quantity as the copper miner did when he made 2% copper ore highly profitable where formerly 5% ore was considered worthless. As a counterpart, this oil shale miner can attain mobility by mining enough lean non- caking material to counteract caking of the richer shale. With mobility he can develop gravity flow and induce a type of particle movement that a volatile heating medium can penetrate uniformly, thereby attaining efficient heat transfer. Following that is development of a large capacity, low unit cost retort, employing the technique of mining by blockcaving.