From North Georgia.
Atlanta, May 21.
--By the train from the front this evening, we learn that a column of the enemy crossed the Etowah river, 8 miles above the railroad bridge, yesterday, marching on Marietta, and McPherson, with 15,000, crossed the night before, 12 miles below Etowah Station, to flank our left.--These movements have made a change of position necessary to our army in order to preserve its communication, and the lines have slowly fallen back along the line of the railroad.
Etowah bridge was burnt last night.
The advance of the enemy in force is become more slow as he recedes from his base.
Several days may elapse before a general engagement can occur.
There has been no skirmishing during the past two days. Portions of the relief committee, that were in the rear, have returned to Atlanta. 