The Block Not Taken

Two blocks replaced some rotting wood,
And sorry I could not lay them both
And be one mason, long I stood
And tamped down one as far as I could
To where it lay in the mortar bed;
Then took the other, as just as level,
And having perhaps the better aim,
Because it was glassy and wanted bevel;
Though as for that the grouting there
Had laid them really about the same,
And both that morning horizontally lay
In scaffold no step had trodden mud.
Oh, I kept the first for another day!
Yet knowing how lay course on the way,
I doubted if I should ever cut wood.
I shall be troweling this with a sigh
Somewhere ages and ages hence:
Two blocks replaced rotting wood, and eye-
saw the one less mortared by,
And that this mud all the difference.