640 MR ARCHIBALD GEIKIE ON THE CHRONOLOGY 
clear up the relations of the Upper and Lower Silurian series and their igneous rocks 
is that which lies between West Linton, in Peebleshire, and the town of Lanark, 
and stretches southward to the flanks of the Silurian hills. When that district is 
worked out we shall probably know whether the felstones of the south of Scot- 
land are referable to two separate periods of movement as I have suggested, or 
should be classed with the Old Red Sandstone igneous rocks of Lanark and Ayr- 
shire. In the meantime, there is no direct evidence in Scotland of igneous action 
during the upper Silurian period. The oldest Scottish trappean rocks now visible 
at the surface, and of which the geological date is certainly known, belong to 
the Old Red Sandstone period ; although it may be possible to prove that part 
at least of the felspathic protrusions in the great Silurian region of the south 
were produced during or even previous to the Upper Silurian period. 
The era of the Old Red Sandstone, almost from its commencement to its close, 
was characterised in Scotland by the frequency and magnitude of its igneous erup- 
tions. Throughout the southern part of the country, as already remarked, there 
is proof, that after the accumulation of the lower, and it may be of the middle 
zone of that formation, a marked, and occasionally even a violent, tilting of the 
strata took place. This movement seems to have been accompanied by the extru- 
sion of much igneous matter in the form of veins, dykes, and amorphous masses. 
Such are the rocks in the neighbourhood of Lesmahagow, and they not improbably 
occur over a wide stretch of country. These felspathic rocks may have been 
produced at some depth below the surface, and facts are not wanting to lead one 
to suspect that in not a few instances they are the results rather of a metamor- 
phism of the adjacent rocks, than of actual protrusion from an independent mass 
of melted matter below. Be this, however, as it may, the date of their production 
is clearly fixed as intermediate between the Lower and the Upper Old Red Sand- 
stone. 
Viewing these felspathic protrusions of the south of Scotland as a whole, one 
cannot fail to be struck by the evidence of their comparatively deep-seated 
origin. There may have been instances over that wide area where the igneous 
forces found a vent for themselves at the surface, and poured out these melted 
rocks and ashy ejections. The Cheviot Hills, and one or two groups of minor 
eminences in the south-west of Berwickshire, may possibly have had such an 
origin. But the immense numbers of dykes and veins of felstone that traverse 
the Lower Silurian and Lower Old Red Sandstone districts of the south of Scot- 
land, must undoubtedly be regarded as having been protruded and cooled, not at 
the surface, but at a considerable depth below it. 
When we pass to the north side of the great Carboniferous belt that bisects 
the island, we find abundant evidence that the same powerful igneous action 
which characterised the southern part of the country also exhibited itself 
north of the Forth in great activity, during the Lower Old Red Sandstone period. 

