That these interstratified calcareous and muddy strata were laid down in water of some considerable depth may be inferred from their general lithological characters. The dark carbonaceous aspect of the limestones points to the probable intermingling of much decayed vegetation with the remains of the calcareous organisms of which these strata chiefly consist. The thin unimportant bands or partings of dark shale show that only the finest muddy sediment reached the quiet depths in which the strata were deposited, while the macerated fern-fragments suggest a long flotation and ultimate entombment of terrestrial vegetation borne seawards from some neighbouring land. [Illustration: Fig. 186.--Section of intercalated dark limestone, shale and chert in the tuff south of Poyll Vaaish Bay, Isle of Man. 1. Limestones and shales; 2. Chert; 3. Tuff.] The cherty bands and nodules, like the flints of the chalk, bear their testimony to the quiet character of the sedimentation in rather deep water beyond the limits within which the sediment from the land was mainly accumulated on the sea-bottom. The origin of these siliceous parts of the series of deposits has still to be investigated. Whether or not they are to be referred to organic causes like chalk-flints, and the radiolarian cherts of the Lower Silurian system, they furnish a fresh example of the remarkable association of such siliceous material with volcanic phenomena, which has now been observed in many widely separated areas all over the world. If we next turn to the stratification of the tuffs, we obtain further evidence of undisturbed conditions of deposition on the sea-floor. The bedding of these volcanic masses, though distinct, appears for the most part to be due rather to the eruption and settlement of alternately finer and coarser detritus than to any marked drifting and rearrangement of these materials by current-action into different layers. Throughout the series of tuffs, indeed, there is, on the whole, a notable absence of any structure suggestive of strong currents or of wave-action in the dispersal and reassortment of the volcanic detritus. The ashes and stones were discharged in such a way as to gather irregularly over the sea-floor into ridges and hollows. There does not seem to have been sufficient movement in the bottom water to level down these inequalities of surface, for we find that they remained long enough to allow twelve feet or more of calcareous and siliceous ooze to gather in the hollows, while the intervening ridges still stood uneffaced until buried under the next fall of ashes. At rare intervals some transient current or deeper wave may have reached the bottom and spread out the volcanic detritus lying there. Such exceptional disturbances of the still water are not improbably indicated by occasional well-defined stratification, and even by distinct false-bedding, in certain finer layers of tuff.