154 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



farther we should find some few fragmentary outliers of the Permian 

 lavas capping the Upper Coal Measures ; and if we merely crossed 

 from the Nith into the tributary valley of the Carron "Water, we 

 should see preserved in that deep hollow a great series of Permian 

 lavas, tuffs, and agglomerates. It is only by a happy accident that 

 here and there these superficial volcanic accumulations have not 

 been swept away. There was probably never any great thickness 

 of them, but they no doubt covered most, if not all, of the district 

 within which the vents are found. 



The Permian necks are, on the whole, smaller than those of the 

 Carboniferous Period. The largest of them probably do not exceed 

 4000 feet in longest diameter, and very few approach that size. 

 The smallest are twenty yards, or even less, in diameter. They 

 generally rise as prominent rounded, dome-shaped, or conical hills, 

 which, as the rock comes close to the surface, remain permanently 

 covered with grass. They thus form a distinctive feature in the 

 scenery of the districts where they occur, as may be well seen in 

 the Dalmellington Coal-field. 



As in those of older geological periods, the necks of this series are, 

 as a whole, irregularly circular or oval in ground-plan, but some- 

 times, like those of the Carboniferous system already referred to, 

 they take curious oblong shapes, and occasionally look as if two 

 vents had coalesced. Here and there also the material of the vents 

 has found its way between the walls of a fissure or the planes of the 

 strata, so as to appear rather as a dyke than as a neck. The necks 

 descend vertically through the rocks which they pierce, having thus 

 the form of vertical columns of volcanic material ending at the 

 surface in grassy rounded hillocks or hills. 



In almost all cases the necks consist of a coarse agglomerate, 

 generally rather red in colour, made up of blocks of such lavas as 

 form the bedded sheets, together with fragments of the stratified 

 rocks through which the chimneys have been blown out. This 

 material is unstratified, but, when it is replaced by less coarse tuff, 

 a rude stratification may often be noticed, the dip being irregularly 

 inward at high angles towards the middle of the vent.^ 



Occasionally some form of molten rock has risen in the funnel 

 and has partially or wholly removed or concealed the agglomerate. 

 This feature is especially noticeable among the necks that pierce the 

 Dalmellington Coal-field. . Portions of basic lavas traverse the 

 agglomerate or intervene between it and the surrounding strata, as if 

 1 Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 463. 



