An attentive examination of these interpolated sedimentary deposits affords some interesting information as to the submarine conditions in which the eruptions took place. The intercalations, sometimes 12 feet or more in thickness, consist mainly of dark limestones, enclosing the usual Carboniferous Limestone fossils; black shales, sometimes showing very fragmentary and much macerated remains of ferns and other land-plants; and black impure argillaceous chert or flint, arranged in bands interposed between the other strata, and also in detached lumps and strings. The dark flaggy limestones and black shales may be paralleled lithologically with those of Castletown and Poyll Vaaish. Indeed, there seems to be little doubt that they represent the contemporaneous type of marine sediment that was gathering on the sea-floor outside the volcanic area, and which during intervals of quiescence or feeble eruptivity spread more or less continuously into that area. The thick mass of tuff must thus have been strictly contemporaneous with a group of calcareous muddy and siliceous deposits which gathered over the bottom beyond the limits of the showers of ashes. [Illustration: Fig. 185.--Section of tuff, showing intercalations of black impure chert, west of Closenychollagh Point, near Castletown, Isle of Man.] One of the most singular features of these sedimentary intercalations is the occurrence of the black cherty material. It may generally be observed best developed at the bottom and top of each group of included strata. Looking at the lumps of this substance scattered through the adjoining tuffs, we might at first take them for ejected fragments, and such no doubt may have been the derivation of some of them. But further examination will show that, as a rule, they are of a concretionary nature, and were formed _in situ_ contemporaneously with or subsequent to the deposition of the tuffs. The accompanying section (Fig. 185) represents the manner in which the chert is distributed through two or three square yards of tuff overlying one of the calcareous groups. The material has been segregated not only into lumps, but into veins and bands, which, though on the whole parallel with the general stratification-planes of the deposits, sometimes run irregularly in tongues or strings across these planes, as shown in Fig. 186, where the dark chert band which overlies the limestones and shales sends a tongue upwards for several inches into the overlying tuff.