426 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
1. AecJcs of JYon-volcanic Debris . — In a few instances the orifices of 
eruption have been filled up entirely with non-volcanic debris. They have 
served as funnels for the discharge of e.xplosive vapours only, without the 
expulsion of any solid volcanic materials. At least no trace of fragmentary 
lavas is met with in them, nor are any beds of tuff or lava intercalated 
ainong the surrounding strata. Some interesting examples of this kind 
were laid hare in the open ironstone-workings near Carluke in Lanarkshire. 
They were circular in gi’ound-plan, descended vertically into the strata, and 
were somewhat wider at the top of the quarry than at the bottom. They 
were filled with angular pieces of Carboniferous sandstone, shide, limestone, 
ironstone and other rocks, these materials being rudely arranged wnth a 
ilip towards the centre of the neck, wdiere the blocks wei’e largest in size. 
Though no fragments of igneous rocks w'ere observed among the debris, a 
few’ string-like veins of “ white trap,” or altered basalt, were seen to traverse 
the agglomerate here and there. The necks and the strata surrounding 
them were highly impregnated with pyrites and sulphate of lime.^ 
A vent of the same nature, but on a much larger scale, has been 
mapped by Mr. Peach in the south of Perthshire, near East Grange, where 
1. Three feet coal ; 
Ontalve coal ; 3. Upper and Lower Black-band Ironafones ; 4. Index Limestone • 
Coal and Janet Peat Coal ; 6. Calmy Limestone ; 7. Xeck. 
it lises thiough the higher coal-bearing part of the Carboniferous Limestone 
seiies (Tig. 14b). It has been encountered in the mining of coal and iron- 
stone, and its cross-section has been ascertained in the underground w’ork- 
ings which have been carried up to its margin. It measures 1500 feet in 
diameter from east to west and 2000 feet from north to south. It does 
not appear ever to have emitted any ashes or lava. Mr. Peach found it 
filled with dark sandy crumbling clays, full of fragments of sandstone, shale 
and coal. Ihese sediments are arranged in layers that dip in the same 
general direction as the strata surrounding the vent. They contain abun- 
dant calcareous nodules of all sizes from that of a hazel-nut up to concretions 
18 feet in diameter. The clays likewise include many of the common shells 
and ciinoids of the Carboniferous Limestone sea, and the same fossils are 
enclosed in the nodules. A remarkable feature in this vent is the occur- 
’ Prof. Jas. Geikie, Mem. Geol. Sxirv. Scotland, Explanation of Sheet 23, p. 39. 
