94 
TKE GEOLOGIST. 
sented by the Secondary formations is involved in darkness. On the 
east and west coasts of Scotland rocks of Secondary age occur north 
of the Grampians, but of the physical history of Central Scotland 
during the time these rocks were deposited, we know nothing. From 
the time of the deposition of our Upper Coal-beds, or, it may be, of 
some Permian Sandstones, up till the time when the whole island 
was locked fast in one immense mantle of ice, we are almost entirely 
ignorant of what was going on in that part of the country which lies 
between the Grampians and the Forth. And the man who shall de- 
cipher for us the physical geogmphy of that period, and reveal to us 
the old surface of that district, with its vegetation and animal pro- 
ductions, prior to the time of the Boulder Clay, will have rendered 
no small service to the cause of Scottish geology. 
But although we have not as yet been able to trace the old surface 
of the land, we are not altogether without data to guide us in our 
researches. One thing is clear and certain, — a great change was 
taking place over the whole face of this region. 
Both during and after the deposition of the coal-measures, great 
volcanic agencies were at work. In the neighbourhood of Edinburgh 
they were particularly active. They had been so during the Old Eed 
Sandstone period, and contributed in no small degree to the forma- 
tion of the Pentland Hills. But towards the close of the Old Eed 
period and the commencement of that time when our Lower Carbo- 
niferous Sandstones were being formed, the Pentland ridge began to 
sink under those Carboniferous seas. The crater of eruption, which 
had been so active in the Pentland area, and produced that varied 
mass of material now presented to us as sheets of felstone and ashy 
bands, became gradually quiescent, till at last it was entirely covered 
over with the increasing deposits. But, while these deposits were 
being accumulated, the igneous forces broke out again at Arthur's 
Seat. Most of the rocks which now form the eastern part of the hill 
belong to this period. The greenstone of the Long Row, the red 
nodular ash and the thin bands of red ashy sandstone to be seen in a 
section at the south side of the Queen's Drive, the green felspathic 
ash of the Dry Dam, the black columnar basalt above it (which also 
constitutes the crag on which stands St. Anthony's Chapel), the two 
basalts behind St. Anthony's, and which form the lower part of 
Dunsapie Hill and Calton Hill, and lastly, that group of felstones 
which form the remainder of the eastern part of the hill from the 
tojp of Dunsapie across Whinny Hill to St. Margaret's Station, and 
which make up most of the Calton Hill, — all these igneous productions 
were contemjporaneous with the Lower Carboniferous strata, and are 
iom\d. interhedded with them. Consequently, all these traps are 
subaqueous. Layers of sediment were first formed, then an eruption 
of lava took place, which spread itself in sheets over the strata ; 
then more layers of mud and sand were deposited by the ocean, and 
then more sheets of lava ejected by the igneous agency below. It 
is evident, moreover, that these traps must have been more or less 
horizontal, for if they had been poured over an inclined surface, 
