Into the Unknown by John Russell Fearn
A laboratory accident suddenly sends four modern-day humans backwards in time, their individual organic bodies almost untouched by the phenomenon, while the physical world around them gradually retreats through the ages. Onward and onward they careen through the past, until they appear in the vividly green Carboniferous Age, filled with overwhelmingly excessive vegetation, festering swamps, and fetid lagoons. The accumulation of rotted vegetation is everywhere around them, vegetation that will eventually become peat, and then through accumulating pressures and chemical change, hard black coal--coal for a future that has already been destroyed. Paradoxes of time--infinite and complete--surround them. But what can they actually do to save themselves...?