
NATURE 
277 

SECTION C. 
GEOLOGICAL SECTION 
OPENING ADDRESS BY THE PRESIDENT, ARCHIBALD 
GEIKIE, F.R.S. 
INSTEAD of offering to the Geological Section of the British 
Association an opening Address on some special aspect, or 
branch of general Geology, I have thought that it might be more 
interesting, and perhaps even more useful, if I were to lay before 
you an outline of the geology of the district in which we are now 
assembled. Accordingly, m the remarks which I am now about 
to make, I propose to sketch to you the broader features of the 
geological structure and history of Edinburgh and its neighbour- 
hood, dwelling more especially on those parts which have more 
than a mere local interest, as illustrative of the general principles 
of our science. 
It would be as unnecessary, as it would be out of place here, 
to cite the long array of authors who have each added to our 
knowledge of the geology of this di-trict ; and many of them also, 
at the same time, to the broad fundamental truths of Geology. 
And yet it would be strange to speak here of the rocks of Edin- 
burgh without even a passing tribute of gratitude to men like 
Hutton, Hall, Jamieson, Hay Cunninglam, Hibbert, Hugh 
Miller, Fleming, Milne Home, and our late esteemed and vener- 
able associate, Charles Maclaren—men who have made the rocks 
of Edinburgh familiar to geologists all over the world. If, there- 
fore, I make no further allusion to these and other names, it is 
neither that I forget fora moment their claims, nor that I now 
bring forward any new material of my own, but because I wish 
to be understood as dealing with facts which, thanks to the 
labours of our predecessors, have become part of the common 
stock of geological knowledge. 
For the purpose of gaining as clear an idea as may be of the 
rocks among which Edinburgh lies, and of the way in which 
they are grouped together, let us imagine ourselves placed on the 
battlements of the Castle, where, by varying our position, we 
may obtain a clear view of the country in every direction for 
many miles round. To the south-east the horizon is bounded by 
a range of high ground, ri-ing as a long table-land above the 
lowland of Midlothian. ‘That is a portion of the wide Silurian 
uplands of the south of Scotland, forming here the chain of heights 
known as the Lammermuir and Moorfoot Hills. Along most 
of its boundary line, in this district, the Silurian table-land 
descends with tolerable rapidity towards the plain, being bounded 
on its north-west side with a long fault, by which the Carboni- 
ferous Rocks are brought down against the hills. These Silurian 
rocks are the oldest strata of the district ; and it is on their con- 
torted and greatly denuded beds that the later formations have 
been laid down. 
Turning now to the south, we see the chain of heights known 
as the Pentland Hills, striking almost from the very suburbs of 
Edinburgh south-westward in the direction of the Silurian up- 
lands, which they eventually reach in the county of Lanark. This 
line of hills rises along an anticlinal axis by which the broad 
Carboniferous tract of the Lothians is divided into two distinct 
portions. The Pentlands themselves consist, as I shall afterwards 
point out, chiefly of rocks of Old Red Sandstone age, but the 
anticlinal fold along which they rise is prolonged through the 
Braid Hills, and through the Carboniferous ground by the Castle 
Rock of Edinburgh, even as far as the opposite shores of Fife. 
From the Castle we can readily follow with the eye the effects 
of this great dominant fold of the rocks. To the east, we mark 
how the strata dip away eastward from the axis of movement, as 
is shown in the escarpments of Salisbury Crag, Arthur Seat, and 
Calton Hill’s, while, on the opposite or western side, the escarp- 
ment of the wooded hill of Corstorphine, facing towards us, 
points out the westward dip. From the same stand-point we can 
even detect the passage of the arch into Fife, for the rocks about 
Aberdour are seen dipping to the west, while eastward, they 
bend overand dip towards the east, at Kinghorn. 
Although the structure of the district is simple when the 
existence and position of this anticlinal axis is recognised, some 
little complication is introduced by a long powerful fault which 
flanks the axis on its south-eastern side. The effect of this fault 
is to throw out a great part of the lower division of the Car- 
boniferous formations, and to biing the Carboniferous Limestone | 
series in some places close against the Lower Old Red Sandstone 
and its volcanic rocks. Another result has been the extreme 
tilting of the strata, whereby the Limestone series along the east | 
side of the fault, has been thrown on end, and even in some parts 
bent back into a reversed dip. Hence, while on one side of the 
axis, the Limestone series is sometimes only a few hundred yards 
distant from the Old Red Sandstone, on the opposite or north-west 
sidc, the distance is fully eleven miles, the intervening space being 
there occupied by endless undulations of the lower divisions of 
the Carboniferous system. Hence, too, the Millstone Grit and 
Coal-Measures come in along the centre of the Midlothian basin 
a short way to the east of the Pentland axis ; while, on the west 
side, they are not met with till we reach the borders of Stirling- 
shire and Linlithgow. 
Another remarkable and readily observable feature is that on 
the west side of the Pentland ridge, the Carboniferous forma- 
tions from almost their base up to the top of the Carboniferous 
Limestone series, abound in contemporaneous volcanic rocks ; 
while, on the east side, beyond Edinburgh and Arthur’s Seat, 
such rocks are absent until we reach the Garlton Hills, to the 
north of Haddington, where they reappear, but ina very different 
type from that which they exhibit to the west. 
Let us now pass in review the different geological formations 
which come into the district around us, beginning with the oldest 
and ascending through the others, till we reach the superficial 
accumulations, and mark, in conclusion, how far the present sur- 
face features are connected with geological structure. 
[The author then describes the various geological formations of 
the district—Silurian, Old Red Sandstone, and Carboniferous— 
dwelling in particular upon the history of volcanic action in that 
part of Scotland. On this subject he remarks :—] 
Outline of the History of Volcanic Action around Edinburgh 
The oldest volcanoes of this part of Scotland were those which, 
during the time of the Lower Old Red Sandstone, poured out 
the great sheets of porphyrite and the showers of tuff which now 
form the main mass of the range of the Pentland Hills. During 
the same long geological period, volcanic action was rife, as we 
have seen, along the whole of the broad midland valley of Scot- 
land, since to that time we must refer the origin of the Sidlaw 
and the Ochil Hills, part of eastern Berwickshire, and the long 
line of upiands stretching from the Pentland Hills through 
Lanarkshire, and across Nithsdale, far into Ayrshire. 
Of volcanic action, during the remainder of the Old Red Sand- 
stone period, there is around Edinburgh no trace. But early in 
the following or Carboniferous period, the volcano of Arthur’s 
Seat and Calton Hill came into existence, and threw out its tiny 
flows of basalt and porphyrite, and its showers of ashes. From 
that time onwards, through nearly the whole of the interval occu- 
pied by the deposition of the Carboniferous Limestone series, 
the district to the west of Edinburgh was dotted over with small 
cones, usually of tuff, but sometimes emitting limited currents of 
different basalt rocks, more especially in the space between Bath- 
gate and the Forth, where a long bank, chiefly formed of such 
lava-currents, was piled up over and among the pools and shal- 
lows in which the limestones, sandstones, shales, and coal-seams 
were accumulated. To the north, also, similar volcanic activity 
was shown in the Fife tracts nearest the Forth; while eastwards 
between Haddington and Dunbar there lay a distinct volcanic 
focus, where great showers of red felspathic tuff and wide-spread 
sheets of porphyrite were ejected to form a bank over which the 
Carboniferous Limestone series was at length tranquilly deposited. 
Volc.nic activity seems to have died out here betore the close 
of the Carboniferous Limestune period. It remained quiescent 
during the deposition of the Millstone Grit and Coal-measures ; 
at least, no trace of any contemporaneous igneous ejection is 
found in any part of these formations. The intrusive masses of 
various basalt rocks, which here intersect the older half of the 
Carboniferous system, are, in all probability, of Lower Carbon- 
iferous date, connected with the eruptions of the interbedded 
volcanic rocks. The next proofs of volcanic action in this 
neighbourhood are furnished by the upper part of Arthur’s Seat. 
At that locality we discover that after more than 3000 feet of 
strata had been removed by denudation from the Pentland anti- 
clinal fold so as to lay bare the old Lower Carboniferous volcanic 
rocks of Edinburgh, a new focus of eruption was formed, from 
which were ejected the basalts and coarse agglomerates of 
the summit and shoulders of Arthur’s Seat. There is no 
trustworthy evidence for fixing the geological date of this 
eruption. Evidently, from the great denudation by which it was 
preceded, it must belong to a much later period than any of 
the Carboniferous eruptions. Yet, from the great similarity of 
| the Arthur’s Seat agglomerate, both in composition and mode of 
| occurrence, to numerous “*necks” which rise through all parts 

