CHAP. XXIII 
THE CARBONIFEROUS SYSTEM IN SCOTLAND 
361 
I In the basin of the Firth of Forth, below the Hui let Limestone, comes a varied 
rj. series of white and yellow sandstones, black shales (oil-shales), cyprid shales 
§ and limestones (Buvdichouse), and occasional coal-seams (Houston), having a 
I total depth of about 3000 feet. This local group abounds in fossil plants, 
~ entomostraca and ganoid fishes. It passes down into the Cement-stone group, 
which, however, is feebly developed in this district, unless it is piartiy re- 
presented by the sandstones, shales, limestones and coals just mentioned. 
Cement-stone group consistilig of red, blue and gi'een marls and shales, red and 
grey sandstones, and thin bauds of cement-stone ; fossils scarce. 
Reddish and grey sandstones and shales, with occasional plant-remains, passing 
down into the deep red (sometimes yellow) sandstones, red marls and corn- 
stones of the Upper Old Bed Sandstone. 
From this table the gradual geographical evolution of the Carlionilerous 
period in Scotland may lie gleaned, observe that at the hegiiming, 
the conditions under which the Old lied Sandstone had been accumu- 
lated still in jiart continued. The great lacustrine basins of the Lower 
Old Eed Sandstone had indeed been effaced, and their sites were occu- 
pied by comparatively shallow areas of fresh or brackish water in which 
the Upper Old lied Sandstone was laid down. Their conglomerates and 
sandstones had been uplifted and fractured. Their vast ranges of volcanic 
material, after being deeply buried under sediment, had been once more laid 
bare, and extended as ridges of land, separating the pools and lagoons 
which they supphed with sand and silt. This singular topography had not 
been entirely effaced at the beginning of the Carboniferous period, for we 
find that many of the ridges which Ijounded the basins of the Upper Old 
Ked Sandstone remained as land until they sank beneath the waters in 
which the earliest Carboniferous strata accumulated. Thus, while no trace 
of an unconformability has yet been detected at the top of the Upper Old 
lied Sandstone, there is often a strong overlap of the succeeding deposits. 
At the south end of the Pentland Hills, for example, the Tipper Old Eed 
Sandstone attains a thickness of 1000 feet, but only three miles further south 
it entirely disappears, together with all the overlying mass of Calciferous 
Sandstones, and the Carboniferous Limestone then rests directly on the 
Lower Old Eed Sandstone. Again, at the north end of the same chain the 
upper division of the Old Eed Sandstone dies out against the lower, which is 
eventually overlapped by the Calciferous Sandstones. 
The change from the physical conditions of the Scottish Old Led Sand- 
stone to tliose of the Carboniferous system was no doubt gradual and slow. 
The peculiar icd sandy sediment continued to be laid down in basins that 
were apparently being gradually widened by access of water from the open 
sea. Yet it would seem that in Scotland these basins still for a long time 
continued saline or, from some other cause, unfavourable to life ; tor the red, 
blue and green shales or marls, and occasional impure limestones or cement- 
stones and gypseous layers, which were deposited in them, are in general 
unfossiliferous, though drifted plants from the neighbouring land are here 
I Tlie Calciferou.s Sandstones are the .stratigrarhical equivalents of the Limestone Sliale and 
lower portion of the Carboniferous Limestone of England. 
