THE SILURIAN VOLCANOES 
BOOK IV 
198 
volcanic outburst, and that such an occurrence took place several times in 
succession over the same area. 
Tliese facts derive further interest from the organic origin of the chert. 
It is now some years since Mr. Peach and his colleagues observed that between 
the Glenkiln Shale with its Upper Llandeilo graptolites and the top of the 
volcanic group in the centi’al part of the Silurian ujilands, alternations of 
green, grey or red shaly mudstones and flinty greywackes are interleaved 
with fine tuffs, and are specially marked by the occurrence in them of 
nodules and bands of black, grey and reddish chert. This latter substance, on 
being submitted to Dr. Ilinde, was found by him to yield twenty-tliree new 
species of Eadiolaria belonging to twelve genera, of which half are new. It 
thus appears that during the volcanic activity there must have been intervals 
of such cpuescence, and such slow, tranquil sedimentation in clear, perhaps 
moderately deep water, that a true radiolarian ooze gathered over the 
sea-bottom.^ 
That tlie deposition of this ooze probably occupied a prolonged lapse of 
time seems clearly indicated by the evidence of the fossils that occur below 
and above the cherts. The graptolites underneath indicate a horizon in the 
Middle Arenig group, those overlying the cherts are unmistakably Upper 
Llandeilo. Thus the great depth of strata which elsewhere constitute the 
Upper Arenig and Lower and Middle Llandeilo subdivisions is here repre- 
sented by only some 60 or 70 feet of radiolarian cherts. These fine 
siliceous, organic sediments probably accumulated with extreme slowness in 
a sea of some depth and over a part of the sea-floor which lay outside the 
area of the transport and deposit of the land-derived sediment of the time.^ 
As an illustration of some of the characteristic features in the succession 
of deposits in the volcanic series of the south-west of Ayrshire, the accom- 
panying section (Fig. 54) is inserted. In descending order we come first upon 
a group of greywackes and grey 
shattery mudstones (a), followed by 
grey-green and dark banded cherts, 
containing Eadiolaria and much pli- 
cated. Next comes a group of dark- 
grey, black and red cherts, with 
numerous partings and thin bands 
of tuff and volcanic conglomerate (c). 
The siliceous bands were certainly 
deposited during the volcanic erup- 
rugose, slaggy upper surface of the 
These lavas have the sack-like 
Fig. 54. — Section of part of the Arenig volcanic 
group, stream south of Beiniane Head, Ayrshire. 
tions, and they are moulded round the 
band of lavas (d) on which they directly lie. 
or pillow structure already described, and they enclose lumps of chert con- 
taining Eadiolaria. A few yards to the west of the line of section bands of 
nodirlar tuff are interposed between the top of the lavas and the overlying 
cherts, with which also they are interstratified. These tuffs contain blocks 
^ Ann. Mag, Nat. Hist. (1890), 6th ser. vi. p. 40. 
^ Annual Report of the Geol. Surv. for 1895, p. 27 of reprint. 
