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
