The volume of water delivered by the stream from the cavern, doe not seem to be above one-twentieth that of the main open stream of the river below, which receives no tributary of importance between this and the other end of the subterranous duct, at Polla.
Close to this cavern, the northern end of the gorge commences, through which the open stream forces its way—a jagged, wall-sided cleft, betwixt precipices, of rather soft, ill-bedded, and cretaceous looking limestone, nearly white in fresh fracture, and whose mean height is probably about 800 feet.
Standing upon the left bank of the river, and 50 feet or so below the level of St. Michael's Cavern, one is enabled to see With some clearness, the geological relations of the main valley and of its lateral ranges, which in transverse section here (looking southward) are approximately shown in Fig. 154. The Apennine limestone of Monte Alburno and its associated range, dips to the south-west and south, but with a constantly varying angle of dip. What is the connection of the beds, of the opposite or eastern chain, with those at the bottom of the valley, is not traceable; it may be one of disunion and dislocation, as the existence of the gorge of Campostrina would suggest; but the beds look, upon the whole, to be parallel continuations at the east side, of those deep under Monte Alburno upon the west.
Above these in the valley, lies the coarse calcareous breceia, in beds approaching conformability at the east side; but where visible through the telescope, at the summits of the underlying ranges, on the west side, appear to be wholly unconformable to the escarpments of Alburno. Small but irregular valley bottoms, are formed at both sides, between
