When the engineers reported that the fortress on El Peñon could not be carried without a loss of one-third the army, Scott decided to move by the south of the lakes; and Worth accordingly advanced, leading the van, as far as San Augustin, nine miles from the city of Mexico. There a large field of lava--known as the Pedregal--barred the way. On the one side, a couple of miles from San Augustin, the fortified works at San Antonio commanded the passage between the field and the lake; on the other the ground was so much broken that infantry alone could advance, and General Valencia occupied an intrenched camp, with a heavy battery, near the village of Contreras, three miles distant. Scott determined to attack on both sides, and sent forward Worth on the east and Pillow and Twiggs on the west. The latter advanced as fast as possible over the masses of lava on the morning of the 19th, and by 2 P.M. a couple of light batteries were placed in position and opened fire on the Mexican camp. At the same time, General Persifer Smith conceived the plan of turning Valencia’s left, and hastened along the path through the Pedregal in the direction of a village called San Jeronimo. Colonel Riley followed. Pillow sent Cadwalader’s brigade on the same line, and later in the day Morgan’s regiment was likewise despatched toward that point. They drove in the Mexican pickets and skirmishers, dispersed a few parties of lancers, and occupied the village without loss. Seeing the movement, Santa Anna hastened to Valencia’s support with twelve thousand men. He was discovered by Cadwalader just as the latter gained the village road; and, appreciating the vast importance of preventing a junction between the two Mexican generals, that gallant officer did not hesitate to draw up his brigade in order of battle. So broken was the ground that Santa Anna could not see the amount of force opposed to him, and declined the combat. This was all Cadwalader wanted. Shields’ brigade was advancing through the Pedregal, and the troops which had already crossed were rapidly moving to the rear of Valencia’s camp. Night, too, was close at hand. When it fell, Smith’s, Riley’s, and Cadwalader’s commands had gained the point they sought. Shields joined them at ten o’clock; and at midnight Captain Lee crossed the Pedregal, with a message from General Smith to General Scott, to say that he would commence the attack at daybreak next morning.