OF THE TRAP-ROCKS OF SCOTLAND. 645 
sive and interbedded, and as greenstones, basalts, felstones, and ashes. Moreover, 
they are found intercalated in nearly every horizon throughout the system, from 
the top of the Old Red Sandstone up to the top of the flat coals which, in the 
Lothians, form the highest part of the series. Every district has its dykes or 
hills of greenstone and basalt, and there are perhaps few areas of any consider- 
able size which do not show traces of one or more distinct centres of eruption, 
by the beds of ash and melted trap intercalated among unaltered strata. As an 
illustration of these features, reference may be made to the area of the Lothians. 
There a continuous series of ashes, with associated greenstones and basalts, 
may be traced from the top of the Old Red Sandstone up into the upper coals of 
Stirlingshire. In this district there is no large group of the Carboniferous rocks 
without, in some locality, its traces of contemporaneous trap. Such traces are 
not, however, found over the entire area. On the contrary, they possess a 
markedly local character, so that a series of beds, which at one point displays 
interbedded trap, may exhibit it nowhere else, or may show at another distant 
point traces of another distinct series of eruptions. These features will perhaps 
be most clearly understood if we trace in outline the geological history of 
the region during the deposition of the Carboniferous rocks, marking, as we pass 
on, some of the principal volcanic ejections by which the succession of the series 
was diversified.* 
At the dawn of the Carboniferous system the area now forming the basin of 
the Forth appears to have been covered by a broad but shallow firth, into which 
a variety of currents carried a constant burden of sand and silt. The sandstones 
and marly shales thus formed have yielded a few shells, as Myalina, Anthracosia, 
Edmondia, and Athyris, along with the spines of Ctenacanthus, and a consider- 
able admixture of vegetable remains. This early period was marked by at 
least one centre of volcanic eruptions,—that of the trappean hills of Edinburgh. 
Streams of various lava-form traps and showers of ash were ejected at successive 
intervals, the pauses being marked in some cases by intercalated seams of sand- 
stone and shale, with land plants and fish remains. After a mass of igneous 
matter, some 700 or 800 feet thick, had been thrown out at this locality, the 
volcanic forces became quiescent, and the old condition of deposition returned. 
Another unimportant eruption took place between Granton and Cramond, and 
then by degrees the firth seems to have silted up into a network of interlacing 
sandbanks, separated by ponds and lagoons of brackish water. The mud-flats 
and shallows nourished a luxuriant growth of the characteristic vegetation of the 
coal period, while the winding channels between had their floor covered with a 
thickening growth of Cyprides, Anthracosic, &c. The congregated remains of 
the Cyprides gave rise to a varying seam of limestone, which, occurring at 
* For the details of part of this region, see the Memoir of the Geological Survey on the 
“Geology of Edinburgh” already referred to. 
VOL. XXII. PART III. 8 E 
