CHAP. XXIX 
THE ISLE OF MAN 
2; 
affords some interesting information as to the submarine conditions m 
which the eruptions took place. The intercalations, sometimes 1 2 feet or 
more in thickness, consist mainly of dark limestones, enclosing the usu.t 
Carboniferous Limestone fossils; black shales, sometimes showing mj 
fragmentary and much macerated remains of ferns and other land-plants ; 
and black impure argillaceous chert or dint, 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 wit 1 
those of Castletown and Poyll Vaaish. Indeed, there seems to he itte 
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 con 
tinuously into that area. The thick mass of tuff must thus have been strict y 
contemporaneous with a group of calcareous muddy and siliceous deposits 
which gathered over the bottom beyond the limits of the showers of ashes. 
One of the most singular features of these sedimentary intercalations 
is the occurrence of the black cherty material. It may genera 5 "® 
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 sue 1 
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 natme, 
and were formed in situ contemporaneously with or subsequen 0 re 
deposition of the tuffs. The accompanying section (Fig. 185 ) represen s 
Fig. 185 , — Section of tuff, showing intercalations of black impure chert, west of Closenj S 
Point, near Castletown, Isle of Man. 
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, thoug 1 on 
the whole parallel with the general stratification -planes of the deposits, some- 
times run irregularly in tongues or strings across these planes, as shown m 
b%. 186 , where the dark chert band which overlies the limestones ai 
R hales sends a tongue upwards for several inches into the overlying u 
