652 MR ARCHIBALD GEIKIE ON THE CHRONOLOGY 
The age of these dykes can of course only be approximately reached. One 
cannot fail to observe, even at the outset, that they all point towards the great 
volcanic centre of the Inner Hebrides. Further, as we examine the country more 
closely, we discover that they increase in number as they are traced from south- 
east to north-west; that is, towards this volcanic area. I have already shown 
that the igneous rocks of Skye and the adjoining islands are later than the lias, 
and are probably of the age of the Middle Oolite. It seems to me, therefore, a 
reasonable inference, that the long north-west and south-east dykes which range 
from these islands across central and southern Scotland may be, partly at least, 
if not entirely, of Oolitic age. And this inference is strengthened by the curious 
fact, that when we trace one of these long dykes out of the palzeozoic rocks of the 
Border country into the northern counties of England, we find it traversing 
liassic and lower oolitic strata. Thus, at both ends of the series, Secondary 
rocks exist, and at both localities they are cut through by the prolongation of 
the series of north-west and south-east dykes which occur in the central and 
southern Scottish counties. 
In conclusion, the points in Scottish geology which I have endeavoured to 
prove in this communication are,— 
1. That in the metamorphosed Lower Silurian region of the Highlands, there 
appears to exist no trace of interbedded igneous rocks, but that in the less altered 
equivalents of these Lower Silurian strata, in the south of Scotland, there are 
traces, if not of the actual eruption of felspathic rocks, at least of their existence 
at the surface during the Lower Silurian period. 
2. That the vast number of felspathic dykes in the south of Scotland may 
perhaps be referred to two geological ages,—one prior to the deposition of the 
Upper Silurian series (but this requires further evidence); the other, between the 
Lower and the Upper Old Red Sandstone, as is shown by the hills of Lesma- 
hagow. 
3. That while, in the south of Scotland, during the earlier ages of the Old Red 
Sandstone period much, if not all of the igneous activity went on at some depth 
below the surface; in the central counties, it manifested itself in the eruption 
of vast sheets of melted lava, and showers of ash and scoriz; that these volcanic 
materials occur both in the lower and upper zones of the formation, and have 
given rise to chains of hills, as the Sidlaws, the Ochils, the Pentlands,, the Campsie 
Hills, and the range of high ground extending for many miles to the south-east 
of Greenock. 
4. That after the cessation of the volcanoes of the Upper Old Red Sandstone, 
and the deposition of the earlier part of the Carboniferous series, the igneous 
agencies broke out again, not however to the same extent as during the previous 
period; that the Carboniferous era in Scotland was characterised by the abun- 

