earth has been converted by a process of partial decay into bitumen. This is called the 'indigenous' theory. Others think that the natural heat of the crust of the earth generated by pressure and, perhaps, other causes, has distilled bitumens from pyrobituminous minerals, and, in some instances, from coal, and they have penetrated the surrounding and overlying porous formations, often filling crevices and forming veins, when the pressure becomes sufficient to rupture the overlying formations. I am inclined to think this latter theory, of 'distillation,' will best account for all the varying conditions under which the various forms of bitumen occur.
Bitumens occur in all periods of the geological history of the earth's crust, but are mainly confined to the formations anterior to the coal period and to the later formations of the tertiary. While asphaltum is found in some of the oldest formations, the greater number of the deposits of solid bitumen and bituminous rocks occur in the more recent formations.
In order to show graphically the relations of the pyrobituminous minerals to the various forms of bitumen, I have arranged the following table, which represents the development of our present knowledge of these substances from the time when M. Leon Malo first published a similar table about forty years ago:
