The story of petroleum probably goes back before the dawn of recorded history. In some places, where the rock layers of the earth's crust were broken, petroleum appeared at the surface of the ground. Early man very likely found this and used petroleum pitch or asphalt long before we have written records. We do know from some of the world's oldest writings that asphalt was used as mortar in buildings. We know that pitch was used in calking boats, and that ancient Egyptians used petroleum to grease their chariot axles. In America the native Indians used petroleum for fuel and medicine long before the white man came. The early settlers considered oil a nuisance because it interfered with getting salt brine from wells that had been drilled for that purpose. Actually, the great petroleum industry got its start from bottled medicine oil and the fact that the whale oil used for lighting and lubrication was becoming very scarce.

Q. Underline an expression which suggests that knowledge about the earliest uses of petroleum are purely speculative.