Complex sentence: The distance between First and Second Avenues was 650 feet, but 610 feet was the gap between Second and Third Avenues, while the blocks between Third and Sixth Avenues were 920 feet, and 800 feet from Sixth to 12th.
Split into simpler sentences: The distance between First and Second Avenues was 650 feet, while the gap between Second and Third Avenues was 610 feet. The blocks between Third and Sixth Avenues were 920 feet, and the gap was 800 feet from Sixth to 12th.