ANNIVEKSAET ADDRESS OF THE PRESIDENT, 9I 



felsite (' hornstone '), porphyrite, and even more basic material 

 sucli as fine dolerite, have been intruded into the tuflTs and breccias. 



On the outskirts of the main vent some subordinate necks may be 

 observed, perhaps marking lateral eruptions from the flanks of the 

 great cone. Three of these occur in a line more than half a mile 

 long, running in a south-westerly direction from the southern edge 

 of the vent. The smallest of these measures about 500 feet in 

 diameter; the largest is oblong in shape, its shorter diameter 

 being about 500 feet and its longer about 1000 feet. The mate- 

 rials that fill these lateral vents are coarse agglomerates with veins 

 and irregular intrusions of a fine horny or flinty felsite. 



It will thus be seen that, viewed as a whole, the materials which 

 now occupy the vents of the volcanic chains in the Lower Old Red 

 Sandstone of Central and Southern Scotland are much more acid 

 than the lavas erupted at the surface. In the Pentland district 

 this more acid material was ejected at intervals in abundant 

 discharges of dust and lapilli, while between these successive 

 discharges copious streams of diabasic and andesitic lavas, either 

 from the same or from some closely-adjoining vent, were poured 

 out. Elsewhere the acid volcanic magma, as I have already stated, 

 has likewise been blown out in fragmentary discharges ; but it 

 appears to have seldom or never communicated with the surface in 

 the form of actual outpourings of lava. Throughout the whole 

 region, however, as a closing phase of the volcanic history, the acid 

 magma rose after the outpouring of the more basic lavas and filled 

 such chimneys of the volcanoes as were not already blocked with 

 agglomerate. Probably after these pipes were plugged the final 

 efforts of volcanic energy were expended in the protrusion of the 

 acid material as sills between the bedding-planes of the surrounding 

 rocks, and as dykes and veins in and around the vents. 



3. Sills and Dyhes. — Nowhere throughout the volcanic tracts of 

 the Lower Old Hed Sandstone is there any such development of 

 sills as may be seen beneath the Silurian volcanic sheets of North 

 Wales. Those which occur are most abundant in the Lanarkshire 

 district, to the north-west and south-west of Tinto, and in the south 

 of Ayrshire. From the village of Muirkirk to the gorge of the Clyde, 

 below the Falls, the Upper Silurian and Lower Old Red Sandstone^ 

 strata are traversed by numerous intrusive sheets of pink and 

 yellow felsite, quartz -porphyry, minette, lamprophyre and allied 

 rocks, which are no doubt to be regarded as part of the volcanic 

 phenomena with which we are here concerned. In the south of 



