I20 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The agglomerates that fill up the vents vary greatly in the nature 

 and relative proportions of their constituents. IJsually the in- 

 cluded blocks and lapilli are pieces of porphyrite, like the rocks of 

 the Plateaux. But with these occur also fragments probably de- 

 tached from the sides of the funnels through which the explosions 

 took place, such as pieces of greywacke, sandstone, and shale. 

 Considerable induration may be observed among these non-volcanic 

 ingredients. In some cases, as in that of the occurrence of pieces of 

 granite referred to on p. 114, it can be shown that the stones have 

 been brought up from some considerable depth. In others it i& 

 easy to see that the blocks have slipped down from some higher 

 group of strata now removed from the surrounding surface by 

 denudation. Some striking illustrations of this feature will be cited 

 from the necks in the south of Eoxburghshire (p. 345). 



When the vents have been filled by the uprise of some molten 

 rock, it is generally, as we have seen, of a more acid character than 

 the ordinary lavas of the Plateaux. Usually it consists of felsite, 

 which is commonly of a dull yellow or grey tint, and often contains 

 scattered blebs of quartz. Good examples of this ordinary material 

 may be seen among the remarkable group of necks on either side of 

 the valley north of the village of Strathblane and in those above 

 Bowling. The great necks of the Garlton Hills — Traprain Law, 

 JN^orth Berwick Law, and the Bass Rock — have already been alluded 

 to as trachytic bosses obviously related to the trachytes of the 

 adjacent plateau. Examples occur, however, where the funnels of 

 eruption have been finally sealed up by the rise of much more basic 

 material, and this has happened even in a district where most of 

 the lava-form necks consist of felsite. Thus, in the Campsie Pells, 

 several such bosses occur, of which the most conspicuous forms the 

 hill of Dungoil (1396 feet). Parther west, among the Kilpatrick 

 HiUs, bosses of this kind are stiU more numerous. The felsitic 

 knobs, bosses, dykes, and sills are particularly abundant along the 

 escarpments and among the rocks underlying them all the way 

 from Pintry to the Clyde. The group of necks near Ancrum and 

 Jedburgh is mainly mkde up of olivine-basalts. This more basic 

 composition of itself suggests that these bosses may be connected 

 rather with the puy- than with the plateau-eruptions. 



In not a few examples, the agglomerate of the vents is pierced by 

 a plug or veins of lava-form material. Many illustrations of this 

 composite structure may be observed along the west front of the 

 great escarpments from Pintry to Ardrossan. In that region the 



