A letter frntn Key West sajs the object of Gen. Newton's receut expedition up St. Marks River, Florida, was to clear out rebels, inflict as much damage as possible, aod if practicable capture St. Marks. Though not acuomplisliing all, he was greatly suocessful. After considerable skïrmishing, he Lad a etubborn fight at Natural Bridge, where the rebels were in Btrong force, and well posted. Though in superior numbers, thej were driyen from their position ; but Gen. Newton, owing to disparity of nombers, deemed it beat to return to Key West, which he did. without being troubled by the rebels.