ANNIVERSARY ADDRESS OF THE PRESIDENT, 89 



cone of Tinto may mark another volcanic centre, though in that case, 

 judging from the unconformable position of the overlying porphy- 

 rites, it must belong to an earlier series of eruptions. This hill, con- 

 sisting of felsitic rocks which have been already referred to, rises to 

 a height of 2335 feet above the sea. It forms part of a continuous 

 mass, which runs in an east and west direction for about five and a 

 half miles, with a breadth of about a mile. Some part, at least, 

 and possibly the whole of this oblong mass is in the form of a sill, 

 which dips towards the north. The conglomerates and sandstones 

 that lie almost at the base of the Lower Old Red Sandstone plunge 

 under it on the southern side, and similar sandstones overlie it on the 

 north. If there be a neck, as one might infer from the shape of the 

 hill, its precise limits are concealed. But there were other, though 

 probably smaller, vents in the immediate neighbourhood. One of 

 these may be marked by the felsite boss which overlooks the village 

 of Douglas, four miles to the south-west of the Tinto mass, while 

 another forms the beautiful little cone of Quothquan Hill, which 

 rises from among the porphyrites on the right bank of the Clyde, 

 immediately opposite to Tinto. 



Eut unquestionably the most interesting vent in the whole region 

 is that which lies at the northern end of the chain of the Pentland 

 Hills. The lavas that flowed from the Tinto centre of eruption die 

 out one after another as they are traced north-eastwards, until, in 

 a distance of about 16 miles, the last of them disappears. Imme- 

 diately beyond that limit, as already mentioned, another volcanic 

 band begins towards the north and rapidly increases in bulk, until, 

 after a course of less than 8 miles, it has attained a thickness of 

 some 7000 feet. At these maximum dimensions it forms the great 

 scarped front of the Pentland Hills, which rises into so prominent a 

 feature in the southern landscape of Edinburgh. Its component 

 lavas and tuffs, which I have shown to present so singular a contrast 

 in composition, and which may be distinguished from each other by 

 topographical form and local colour (even when seen from a dis- 

 tance), range in parallel bands from south-west to north-east, 

 until, along the base of the northern wall of the Pentlands, their 

 continuity abruptly ceases. The lower ground, which extends from 

 the foot of that steep declivity to the southern suburbs of Edinburgh, 

 and includes the group of the Braid HiUs, is occupied by another 

 and more complex group of rocks in which the parallelism and 

 continuity, so persistent in the Pentland chain, entirely disappear. 

 When I mapped this district, many years ago, for the Geological 



