The Mexican War, brief as was its period of operations in the field, was marked by many deeds of daring, and also was the scene of the first service in the field of various officers who afterward became prominent in the Civil War. Chief among these were the two great leaders on the opposite sides, General Lee and General Grant. Lee's services in the campaign which Scott conducted against the city of Mexico were especially brilliant, and are likely to be less familiar to the reader than any incident drawn from his well-known record in the Civil War. The most striking among them was his midnight crossing of the lava-fields before Contreras. On the 19th of August, 1847, Scott's army lay in and around San Augustin, a place situated on a branch of the main road running south from the city of Mexico. This road divided into two at Churubusco, the other branch running near Contreras. Between these two roads and a ridge of hills south of San Augustin extended a triangular region known as the Pedregal, and about as ugly a place to cross as any ground could well be. It was made up of a vast spread of volcanic rock and scoriae, rent and broken into a thousand forms, and with sharp ridges and deep fissures, making it very difficult for foot-soldiers to get over, and quite impassable for cavalry or artillery. It was like a sea of hardened lava, with no signs of vegetation except a few clumps of bushes and dwarf trees that found footing in the rocks. The only road across it was a difficult, crooked, and barely passable pathway, little better than a mule track, leading from San Augustin to the main road from the city of Mexico. On the plateau beyond this sterile region the Mexicans had gathered in force. Just beyond it General Valencia lay intrenched, with his fine division of about six thousand men and twenty-four guns, commanding the approach from San Augustin. A mile or more north of Contreras lay General Santa Anna, his force holding the main city road. Such was the situation of the respective armies at the date given, with the Pedregal separating them. Captain Lee, who had already done excellent engineering service at Vera Cruz and Cerro Gordo, assisted by Lieutenants Beauregard and Tower of the engineers, had carefully reconnoitred the position of the enemy, and on the morning of the 19th the advance from San Augustin began, Captain Lee accompanying the troops in their arduous passage across the Pedregal. One of those present thus describes the exploit: