The voice of him that crieth in the wilderness, Prepare ye the way of the LORD, make straight in the desert a highway for our God. Every valley shall be exalted, and every mountain and hill shall be made low: and the crooked shall be made straight, and the rough places plain:
This passage, of which the force may not at once strike the reader, is well illustrated by a description which Diodorus Siculus gives of a march of Semiramis, Queen of Persia.
" marching towards Ecbatana she came to a mountain called Zarkeum. which, extending many furlongs, and being full of craggy precipices and deep hollows, could not be passed without taking a long and circuitous route. Being desirous, therefore, of leaving an immortal monument of herself, as well as to make a shorter way, she ordered the precipices to be cut down, and the hollow places to be filled up with earth; and, at a great expense, she made a plain open road, which to this day is called the road of Semiramis.
Afterwards she made a progress through Persia and all her other dominions in Asia, and wherever she came she ordered the mountains and craggy rocks to be cut down, and at a vast expense made the ways level and plain. On the other hand, in low places she raised mounds."
Something similar to this may possibly have been known to the prophet, and from it his imagery appears to be borrowed.