ANXIVERSARY ADDRESS OF THE PRESIDENT. II9 



are concealed under these plateaux. A few appear at the surface 

 among the lavas and tuffs, but by far the largest number now visible 

 have been revealed by denudation, the escarpments having been cut 

 back so as to lay bare the underlying rocks through which the necks 

 rise. Thus along the flanks of the great escarpment that extends 

 from near Stirling by Fintry and Strathblane to Dumbarton some- 

 where about 50 vents may be counted in a distance of about 16 miles. 

 ;N"owhere in Scotland do such necks form a more conspicuous feature 

 in the scenery as well as the geology than they do between Pintry 

 and Strathblane, where, standing out as bold isolated hills in front 

 of the escarpments, their conical and rounded outlines present a 

 striking contrast to the terraced escarpments behind them. Along 

 the west front of the hills between Gourock and Ardrossan 17 

 agglomerate-vents occur in a distance of 16 miles. In Roxburgh- 

 shire a group of large agglomerate-necks is dotted over the Silurian 

 country around Melrose and Selkirk.^ 



From the evidence of these necks it is plain that the volcanic 

 materials must in each case have been supplied, not from one great 

 central orifice, but from abundant vents standing sometimes singly, 

 with intervening spaces of several miles, often in groups of four or 

 five within a single square mile. The size of the funnels, as measured 

 by the diameter of the necks, varies from less than 100 feet to 

 upwards of a mile.^ In shape they are usually circular or elliptical, 

 but examples occur where the material of the neck extends into 

 an oblong mass a mile or more in length and not many yards in 

 width. Occasionally it can be shown that such long narrow dyke- 

 like masses of agglomerate have risen along lines of fault.^ But 

 in this and in other series of volcanic vents it is remarkable how 

 seldom any relation can be traced between their sites and actually 

 visible lines of dislocation. 



^ In this region and farther southward, besides the plateau-eruptions, a later 

 group of puys is to be seen, and it is difficult to discriminate bet\Yeen the necks 

 belonging to the two groups. Those which lie to the east are probably con- 

 nected with the plateaux, those to the west with the puys. The latter are 

 referred to on p. 135. 



^ In some instances of large irregularly-shaped masses of agglomerate and 

 tuff we not improbably see the coalescence of successive vents nearly on the 

 same site. A large tract of this kind on the liigh grounds of Renfrewshire, 

 referred to on a later page, is probably an example. 



■^ A good illustration of this structure has been mapped by Mr. B. N. Peach 

 near Mosspaul at the head of Ewesdale, in a vent, or perhaps group of vento, 

 which should probably be placed among those of the puys. 



^2 



