136 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



locations of the Carboniferous system are on the whole later than the 

 volcanic phenomena. It may sometimes be observed, however, that 

 the vents are arranged in lines or in scattered groups. A remarkable 

 instance of the linear distribution is furnished by the chain of necks 

 which extends from the Vale of the Tweed at Melrose soath-west- 

 wards across the watershed and down Liddesdale. I shall refer 

 again more particularly to this district. As examples of the 

 arrangement of vents in groups I may cite those of the west of Fife, 

 of West Lothian, and of the north of Ayrshire. 



A convenient classification of the vents may be made by dividing 

 them into four groups according to the nature of the material thatr 

 now fills them : — 1st, necks of non-volcanic debris ; 2nd, necks of 

 tuff and agglomerate ; 3rd, necks of similar materials, but with a 

 central plug or branching veins of basalt; 4th, bosses of basalt or 

 other lava. 



(1) In a few instances the orifices of eruption have been filled 

 up entirely with non-volcanic material. They have served as 

 funnels for the discharge of explosive vapours only, without the 

 expulsion of any solid volcanic materials. At least no trace of frag- 

 mentary lavas is met with in them, nor are any beds of tuff or lava 

 intercalated among the surrounding strata. Some interesting exam- 

 ples of this kind were laid bare in the open ironstone- workings near 

 Carluke, in Lanarkshire. They were circular in ground-plan, de- 

 scended 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, shale, limestone, iron- 

 stone, and other rocks, these materials being rudely arranged with a 

 dip towards the centre of the n(;ck, where the blocks were largest in 

 size. Though no fragments of the igneous rocks were 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 impreo- 

 nated 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 west of Fife, near Grange, where it 

 rises through the higher coal-bearing part of the Carboniferous 

 Limestone series. It measures 1500 feet or more in diameter, and 

 does not appear ever to have emitted any ashes or lava. Mr. Peach 

 found it filled with non-volcanic sediment, arranged in layers dipping 

 at high angles towards the middle of the vent, and among these 



' Jas. Geikie, Mem. Greol. Surv. Scotland. Explanation of Sheet 23, p. 39. 



