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The sketch in the top right-hand corner of the page represents a view Lake Agnano, a former crater lake in the Campi Flegrei (Phlegraean or Burning Fields) which was drained in 1870. The lake was famously the site of the Grotto del Cane (Dog’s Grotto), so-called because dogs were traditionally used to demonstrate the effect of the poisonous fumes which arose from the cave. The vista depicts the view looking north-west across the lake towards the hill of Camaldoli on the left and Vesuvius on the right. The composition continues on the opposite sheet of the double-page spread, see folio 76 verso (D15706; Turner Bequest CLXXXIV 74a). For further related views see folios 75–75 verso and 77 (D15703–D15704 and D15707; Turner Bequest CLXXXIV 73–73a and 75).
The remaining two views on the lower part of the page meanwhile appear to depict Lake Avernus and the ruins of the so-called Temple of Apollo, with Monte Corvara beyond. The dark cave opening on the left-hand side of the larger study represents the so-called Grotta della Sibilla (Grotto or Cave of the Cumaean Sibyl) on the north-west banks of the lake.1 For related views see folio 77 verso (D15708; Turner Bequest CLXXXIV 75a).
Compare a sketch by James Barry (1741–1806), Lake Avernus circa 1769 (Yale Center for British Art, New Haven), reproduced in William L. Pressly, ‘On Classic Ground: James Barry’s “Memorials” of the Italian Landscape’, in Record of the Art Museum, Princeton University, vol.54, no.2, 1995, p.13, fig.1.