24 
THE CARBONIFEROUS VOLCANOES 
Fig. 184. — Limestones passing under strati- 
fied tuffs, Poyil Vanish, Isle of Man. 
exposed on the coast at Poyil Vanish, intercalated in and overlying the 
dark hmestones of that locality (Pig. 184), which have been assigned,^ 
series 1 Th^M ’ \° ^ UPpCr part of the Carboniferous Limestone 
s. The Manx volcanoes may therefore be regarded as having prob- 
ably been in eruption during the later por- 
tion of the Carboniferous Limestone period. 
Owing to irregularities of inclination, 
the thickness of the volcanic group can 
only be approximately estimated. It is 
probably not less than 200 or 300 feet. 
Lut as merely the edge of the group lies 
on the land, the volcanic rocks may reach 
a considerably greater extent and thickness 
under the sea. 
The volcanic materials consist mainly 
n „„u j? i of bedded tuffs, but include also several 
ecks of agglomerate and a number of dykes and sills. So far as I 
lave observed, they comprise no true lava-streams. 2 These Manx tuffs 
preseut many of the familiar features of those belonging to the puy- 
eiup ions of Central Scotland, but with some peculiarities worthy of 
attention. They are on the whole distinctly bedded, and as their 
duration . is generally in a westerly direction, an ascending order can 
be traced in them from the eastern end of the section to the highest 
parts of the group associated with the Poyil Vaaish limestones. Their 
co oui is the usual dull yellowish-green, varying slightly in tint with 
changes m the texture of the materials, the palest bands consisting of the 
finest dust or volcanic mud. Great differences in the size of their frao- 
ZTl C< vJ, Stltl n ntS ma} ' be observed in successive beds, coarse and fine 
bands rapidly alternating, with no admixture of non-volcanic sediment, 
rough occasional layers of fine ash or mudstone, showing distinct current- 
bedding, may be noticed. 
of seamTof H ^ SUeCeSsion of eru P tions are marked by the intercalation 
of seams of limestone or groups of limestone, shale and black impure chert. 
Thev “ 'f , ak w f sometimes c «riousl y local and interrupted. 
anfbZ T r° ° Ut rapidl7 ’ th6reby all0win S the tllff above 
< d below them to unite into one continuous mass. They seem to have 
een accumuiated m hollows of the tuff during somewhat prolonged inter- 
bv a reZT^ q Z CenCe ;- and t0 W been sudd ^T brought to an end 
y a renewal of the eruptions. There are some four or five such inter- 
calated groups of calcareous strata in the thick series of tuffs, and we may 
"Z“ lg ^ ^ P "~ iQ ^ - energy ofZ f 
An attentive examination of these interpolated sedimentary deposits 
1 R. Etheridge jun., in Mr. Horne’s paper above cited. 
in ssss: “ * tws — 1 ** 
