OF THE TRAP-ROCKS OF SCOTLAND. 653 
dance and activity of its volcanic foci—so much so that there is not a well defined 
zone of Carboniferous beds in the Lothians which does not at some point display 
its intercalated sheets of ash or greenstone; but these eruptions were markedly 
local alike in their extent and in the character of their erupted material. 
5. That after the Carboniferous series, there is at present a great gap in the 
chronology of the Scottish trap-rocks, the next known records of volcanic action 
being the greenstone and basalt hills of Skye, Rasay, and the islands to the south, 
which probably belong to the epoch of the Middle Oolite. 
6. That to the same Oolitic period we ought probably to refer the long north- 
west and south-east dykes which range from the Inner Hebrides across Scotland 
to the coast of Northumberland and Durham. 
7. That of Upper Secondary igneous rocks, Scotland has not yet furnished 
any trace; but that in Mull there occur basalts and ash-beds which, from the 
association with them of dicotyledonous leaves, have been referred to the age 
of the Miocene Tertiary. 
8. That the upper part of Arthur’s Seat is long subsequent to the igneous 
rocks on which it rests, and may possibly be of the same age with the Miocene 
lavas of Mull. 
The facts stated in the preceding pages suggest some curious reflections on 
the question whether or not the volcanic forces are deep-seated. Thus the contem- 
poraneous trap-rocks of the Old Red Sandstone are highly felspathic—felstones, 
porphyries, and felspathic ashes and conglomerates. The lava-form traps of the 
Carboniferous series are nearly all augitic—greenstones, basalts, &c. The fel- 
stones as a rule, cover large tracts of country; the greenstones occur only as local 
and sporadic patches. Again, the dykes that traverse the Silurian and Lower 
Old Red Sandstone districts are almost entirely felstones; those in the Carboni- 
ferous tracts are as uniformly basalt or greenstone. These latter dykes are dark 
crystalline rocks; those of the Upper Old Red Sandstone of adjoining tracts (as 
in Haddingtonshire and the Pentland Hills) are dull, compact, and ferruginous. 
In short, we can hardly resist the conclusion, that the nature of the igneous dyke 
has some relation, whatever it may be, to the nature of the rock which the dyke 
traverses. And if this be so, we are led to ask, whether the force which produced 
the dyke may not have been less deeply seated than is commonly sup- posed ? 
In another paper on this subject, I shall endeavour to show, that during the 
accumulation of the Upper Old Red Sandstone and the Carboniferous rocks of 
central Scotland, the subterranean forces operated not only in the eruption of vol- 
canic material, but to a marked degree in the elevation and depression of part of 
this district ; and that these movements were attended by some important changes 
in the physical geography, and in the Fauna and Flora, of that ancient period. 
VOL. XXII. PART III. 8 G 
