ANNIVERSARY ADDRESS OF THE PRESIDENT. 87 



they are certainly younger than the older part of the Upper Silurian 

 series. They must be later, too, than the chief plication of these 

 strata ; but they are older than the basement Carboniferous rocks 

 which contain pebbles of them. Their date of eruption is thus 

 narrowed to the interval between the later part of the Upper Silu- 

 rian period and the close of the Old Red Sandstone. I have myself 

 little doubt that they are to be associated with the volcanic epoch 

 we are now considering, as it was the only known great episode 

 of igneous activity in this region during the interval within which 

 the protrusion of these granites must have taken place. In 

 the Cheviot Hills, indeed, we have evidence of the eruption 

 of a large mass of augite-granitite through the porphyrite-lavas 

 of the Lower Old Red Sandstone, with abundant veins projecting 

 from it into them. Mr. TeaU and Mr. Clough, who have so 

 well described this mass, were inclined to regard it as marking 

 the source whence the lavas and tuifs of the region proceeded.^ It 

 may not improbably have been protruded up one or more funnels 

 which had previously served as orifices for volcanic discharges, and 

 in that case would mark one of the later phases of the volcanic 

 energy of the region. Its position may be suggestively compared 

 with that occupied by the granophyric bosses in the Tertiary vol- 

 canic series of the Inner Hebrides. 



Leaving these granitic masses aside for the present, I would 

 direct attention to the mare important bosses and groups of bosses 

 which, lying well within the volcanic area, appear to mark some of 

 the vents of the basin. These are most numerous along the chain 

 of the Ochil and Sidlaw Hills ; but they probably do not mark the 

 sites of the chief vents of that chain, which appear to be concealed 

 under the Carboniferous rocks of the Midland Yalley. The bosses 

 now visible rise through different portions of the volcanic series, and 

 are therefore not the oldest or original vents of the group. In the 

 south-western part of the chain they are chiefly small in size, 

 seldom exceeding half a mile in diameter, and have been filled 

 sometimes with crystalline, sometimes with fragmental materials. 

 Two bosses, containing the remarkable granophyric quartz-diorite 

 already referred to, emerge from among the tuffs in a low part of the 

 series, immediately above the village of Tillicoultry in Clackmannan. 

 Two or three more, which are occupied by quartz-porphyries, pierce 

 the volcanic group a few miles to the west of Loch Leven. The 



1 TeaU, Geol. Mag. for 1883, p. 106 ; Clough, Mem. Geol. Survey, ' G-eology 

 of the Cheviot Hills,' Sheet 108 N.E., 1888, p. 24. 



</2 



