OUTLINE OF ITS GEOLOGY. 151
flanks the eastern side of the Pentland Hills, they have been thrown nearly od
edge from Portobello southward by Edmonston, Gilmerton, Loanhead, Penicuik,
and Brunston. Some thin bands of limestone with dwarf marine fossils
overlie these coals. One of them is welt exposed at the Joppa quarry.
Overlying this limestone series there is found a great mass of coarse
sandstone and fine conglomerate, often red in colour, and known locally as the
Roslin sandstone or Moor-rock. It attains a depth of 340 feet, and is believed
to be the equivalent of the millstone grit of the English series. It forms the
sides of the romantic ravine of the Fisk between Roslin and Lasswade.
Next in order comes the highest section of the Carboniferous system, known
as the Coal-Measures. It consists of sandstones, shales, fireclays, and coal-seams,
and in the Mid-Lothian basin attains a thickness of 1590 feet. It seems to
have been formed under circumstances not unlike those in which the Edgecoal
series was laid down, with the exception that the marine limestones were
not formed. The swamps were from time to time densely covered with
vegetation, which, though generally agreeing with that of the older series,
differs considerably in many of its species. These thick matted accumulations
of vegetation form now the seams of coal, while the sandy and clayey
strata between them represent the sediment laid down upon the submerged
forests as each of these was successively carried down beneath the water.
At this part of the history we come upon the greatest hiatus in the geological
records of the district. A vast series of ages passed away, during which the
physical geography of the area of Britain went through many vicissitudes,
and the plants and animals alike of land and sea were completely changed.
Yet of these events no geological memorial has been preserved at Edinburgh.
We know from evidence elsewhere existing that long after our coal-fields were
formed some of them were pierced by volcanoes. Those of Ayrshire, Lanarkshire,
and Fife suffered in this way. Probably the upper part of Arthur's
Seat belongs to that period of volcanic activity. At a far later, though still
remote, time, a renewed outburst of the subterranean forces gave rise to the
vast basaltic plateaux of Antrim, Mull, Eigg, Skye, and the Faroe IsLnds.
When these western vents were still busy pouring out their masses of lava, the
country, by some process as yet little understood, came to be cracked across in
innumerable places, the fissures having on the whole an east and west trend, and
increasing in number toward the volcanic district of Antrim and the Hebrides.
Into these fissures the basalt f?om below rose, filling them and forming the
long wall-like masses known as diies. Several of these dikes occur at or
near Edinburgh.' Some are now concealed by the streets of the city. One
152 EDINBURGH PAST AND PRESENT.
traverses the coal-field near Niddry, while two very conspicuous examples
run across the Carboniferous Limestone series near Prestongrange and Longniddry.
Another great gap, for the filling up of whichno evidence exists in this
part of the country, separates these volcanic rocks from the next geological
events in our chronicle-those of the Ice Age. The neighbourhood of Edinburgh
will always bear a special interest in regard to this part of geology, from
the fact 'that it was here Sir James Hall observed and described, those
dressed ' rock-surfaces which are now everywhere acknowledged to be due to
the grindiog action of ice. They are to be seen on the west slope of Corstorphine
Hill, on the top of the southern part of Salisbury Crags, on the sandstone
at Joppa salt-pans, on the porphyrite at Blackford Quarry, on the top of
Allermuir Hill, one of the Pentlands, at a height of 1617 feet, and in many
other places. The general direction of the strk and groovings is a little to the
north of east, indicating that the mass of ice which produced these markings
moved seawards along the line of the valley of the Firth of Forth.. The roqks
of the neighbourhood of Edinburgh pass beneath masses of glacial drift-the
productof the glaciers, icebergs, and seas of the glacial period. At the bottom
of these deposits lies the boulderclay or till-a stiff dark-blue clay stuck full of
stones of all sizes, up to blocks of a yard in diameter. Many of these are well
smoothed, and striated like the surfaces of the solid rocks underneath. On
examination it is found that the majority of them are of local derivation, that
some have come from distances of ten or fifteen miles to the west, a smaller
proportion from western hills twenty or thirty miles away, while a very small
number have travelled from the Highland mountains. Thus the stones
corroborate the testimony of the striz on the rocks, that the general icemovement
was here from the west. The gradual deflection to east-by-north
,was evidently due to the influence of the shape and direction of the great
valley upon the march of the ice.
While the lower parts of the boulderclay appear to have been formed under
a huge sheet of land-ice moving steadily across the country, like the enormous
icemantle of Greenland, the upper parts of the deposit suggest that they may
have originated to some extent in the sea, either underdw solid ice or under
the broken bergs which flanked the long front of the ice-sheet They contain
larger blocks than the lower parts of the till, and these are often arranged in
rude lines.
The boulderclay, as might be expected, is singularly destitute of fossils.
To the west of Edinburgh,; when the Union Canal was being cut, a well
