t 48 EDINBURGH PAST AND PRESENT.
~~
glomerates and sandstones of the Old Red Sandstone system were elaborated.
They seem to have covered a large part of Western Europe, and to have
extended. eastwards over much of the north of Russia. At the beginning’of
the Old Red Sandstone perid, what is now the northern. half of Britain may
have bekn connected with some large continental mass of land, but was certainly
covered by wide sheets of water, out of which the high grounds rose as scattered
groups of islands. This land too was slowly sinking down, so that the waters
encroached more and more’ upon its shores, from which enormous quantities
of gravel and sand were swept away to form the vast piles of conglomerate and
sandstone. Some idea may be formed of the extent of this depression, and
of the amount of detritus which must have been worn off the land, from the
fact that the Old Red Sandstone, even now, after the lapse of so many ages of
subsequent decay, is still often 15,000 feet thick, and in the east of Strathmore
exceeds even 20,ooo feet.
As a rule,’ the waters in which those deposits were laid down seem to have
been rather unfavourable to life, at least organic remains are for the most part
scarce, though here and there fishes occur abundantly. In the immediate
neighbourhood of Edinburgh no fossils have yet been met with in the Old
Red Sandstone except some traces of plants towards the top. In this district,
however, considerable interest belongs to that system, owing to the large masses
of volcanic rocks which are imbedded in it. The conglomerates and sandstones
constitute the southern end of the Pentland Hills and stretch thhnce
into the counties of Peebles and Idnark. But the main mass of these hills,
including also the Braid Hills near Edinburgh, consists of various lavas and
tuffs ekpted from volcanic vents which continued to be active during a long
part of the Lower Old Red Sandstone period, The curious tuKlike masses
of .the Braid Hills, as well as the thickening of the bvas towards the north,
seem to point to one or more chief craters of emission having existed somewhere
about the site of the Braid Hills. Owing to the deposit of later formations
upon them; we cannot tell bow thick these piles of volcanic material may
originally have been. But even now, the part which is seen would, if placed
in its original position, make a mountain-ridge considerably higher than Beb
Nevis, During the time when these Pentland volcanoes were at work there
was a prodigious amount of volcanic activity over Scotland. From the
ridge of the Highlands pormous streams of lava and showers of dust and
stones were thrown into the waters which then spread over the centre of the
country. From the high land of the southern uplands there would seem 10
hatre been a similar outpouring. Many thousands of feet of volcanic rock
