"When Adam Smith explains the fall in the rate of profit from an over-abundance of capital, an accumulation of capital, he is speaking of a permanent effect and this is wrong. As against this, the transitory over-abundance of capital, over-production and crises are something different. Permanent crises do not exist."
Theories of Surplus-Value
http://www.marxists.org/archive/marx/works/1863/theories-surplus-value/ch17.htm