ANNIYERSAEY ADDRESS OF THE PRESIDENT. 1 35 



out above ground. Other cases occur where beds of lava or of tuff, 

 or of both together, have been intercalated in a group of strata, but 

 with no trace whatever of the vent from which they came. The 

 most complete chronicle, preserving at once a record of the outflow of 

 lava, of the showering forth of ashes and bombs, and of the necks 

 that mark the vents of eruption, is only to be found in some of the 

 districts. 



I shall therefore, in this section of my Address, reverse the order 

 of treatment I have hitherto followed, and treat first of the necks, 

 then of the materials emitted from them, and lastly of the sills and 

 dykes. 



1. Vents. — A large number of vents rise through the Carbo- 

 niferous rocks of Scotland. Some of these are not associated with 

 any interbedded volcanic material, so that their geological age can- 

 not be more precisely defined than that they must be later than the 

 particular formations which they pierce. Some of them, as I shall 

 endeavour to show, are in all probability Permian. But there are 

 many which, from their position with reference to the nearest 

 intercalated lavas and tuffs, are almost certainly to be regarded as 

 of Carboniferous age. Those which are immediately surrounded by 

 sheets of lava and tuff, similar in character to the materials in 

 the vents themselves, are without hesitation connected with these 

 sheets as marking the orifices of discharge. 



The vents of the puys never attain the size often reached by those 

 of the plateaux. Their smallest examples measure only a few yards 

 in diameter, their largest do not much exceed half a mile.^ They 

 have seldom any apparent relation to faults. Probably the dis- 



^ The following measurements of necks in the great chain of them which 

 runs in a north-east and south-west direction from Melrose across into 

 Liddesdale are taken from the 6-inch field-maps of the Geological Survey, and 

 were chiefly mapped by Mr. B. N. Peach. A few of those first enumerated pro- 

 bably belong to the plateau series, in which the necks are, on the average, larger 

 than those of the puys. The great neck at Melrose measures 8800 by 4200 feet ; 

 Carewood Rig, on the borders of Roxburghshire and Dumfriesshire, 7000 X 2400 ; 

 the largest of the chain of necks from Linhope to Skelfhill, 4500x500; Black 

 Law, between Bedrule and Jedburgh, 3400x1600; Great moor, near Maiden 

 Paps, Roxburghshire, 2600x2400; Tinnis, Liddesdale, 1500x1000; Rubers 

 Law, 1500x1000; Minto Hill (north), 1500x1100; Minto Hill (south), 

 2300 X 1650. These are the largest necks of the region. The smallest mapped 

 by Mr. Peach are the following : — Pike Law, Arkleton, Tarras Water, 500x500 

 feet; Harwood, Stonedge, 5 miles S.E. from Hawick, 500x300; Arkleton 

 Burn, Dumfriesshire, 400 X 100 ; Roan Fell, Liddesdale, 300 x 200 ; Hartsgarth 

 Burn, Liddesdale, 250 X 200 ; Dalbate Burn, 250 X 120. 



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