The poem contains the phrase "harmonious madness" referring to the beauty of the song of the skylark.  The persona feels that if he had half the gladness within himself that the skylark must have to sing its song, it would produce an ecstacy he calls "harmonious madness."  With this harmonious madness the persona would be able to create such beautiful poetry that the world would listen in awe in the same way that the persona is listening to the skylark.