The idea of juxtaposing two complimentary notions of pastoral - the pastoral eclogue vs. the "pastoral letter" - makes me think of the traditional "wheel of Virgil". This was a medieval description of Virgil's literary project : to begin with simple, arcadian-nostalgic eclogues, proceed to didactic Georgics, and synthesize with heroic-utopian-tragic epic (Aeneid). In this way Virgil circumscribed or synthesized both "pastoral" principles - within a "wheel" which contained all modes of poetic narrative (that is, excluding drama).