CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 443 
succeeding strata. Thus at the south end of the Pentland Hills they attain a 
thickness of upwards of 1000 feet, but only three miles towards the south they 
have entirely disappeared, and the Lower Old Red Sandstone is directly 
covered by the Carboniferous Limestone. 
With the close of the epoch in which these red deposits were accumulated 
a great change took place in the geography of the Forth basin. Concen- 
tration of the water in the enclosed lagoons, and precipitation of iron-oxide 
amid the gathering sand and silt no longer prevailed. The sheets of water 
became more continuous, and were liable at intervals to irruptions of the sea. 
A more copious rainfall, at the same time, may perhaps be inferred from the 
thick zones of white sandstone, occasional bands of fine conglomerate, and 
abundant seams of shale. The constantly varying aspect of the strata must 
at least indicate much more varied climate and conditions of denudation and 
deposition than are presented to us by the monotonous barren red sandstones. 
The muddy floor of the shallow water must, in many places, have supported 
a luxuriant growth of vegetation, which is preserved in the occasional seams 
and streaks of coal. Numerous epiphytic ferns grew on the suberial stems 
and branches of the lycopodiaceous trees. Large conifere clothed the higher 
grounds, from which the streams brought down copious supplies of sediment, 
and whence a flood now and then transported huge prostrate trunks of pine. 
It was during this condition of things, distinct from that which then 
prevailed in the rest of Scotland, that the Carboniferous volcanoes began 
their activity. The basin of the Firth of Forth was gradually dotted over 
with little volcanic cones, and here and there with long volcanic ridges 
formed by the confluence of lavas and showers of tuff. The whole area was 
all the while undergoing a process of slow subsidence. Cone after cone, 
more or less effaced by the waters which closed over it, was carried down 
and buried under the growing accumulation of sediment. But new vents 
of eruption opened elsewhere, throwing out for a time their dust or 
lava-streams, and then lapsing into quiescence as they slowly sank into the 
lagoon. 
The occasional presence of the sea over some portions of the area is well 
shown by the occurrence of thin bands of limestone or shale, containing such 
fossils as Orthoceras, Bellerophon, and Discina. Yet the general estuarine or 
fresh-water character of the accumulations seems satisfactorily established, 
not only by the absence of undoubtedly marine forms from most of the strata, 
but by the abundance of ostracod crustacea (Leperditia), forming sometimes 
thick lenticular seams of limestone, such as might have been formed in 
distinct limited hollows, by the numerous scales, teeth, bones, and coprolites of 
small ganoids, and by the crowded remains of terrestrial vegetation, often 
admirably preserved among the shales. 
