442 PROFESSOR GEIKIE ON THE 
In looking at the chronological distribution of the volcanic masses, we find 
that the true interbedded, as distinguished from the intrusive sheets, belong 
entirely to the two lower subdivisions of the Carboniferous system. They 
begin about the top of the red sandstone series which forms the base of the 
system, and continue at intervals till towards the top of the Carboniferous 
Limestone series, when they entirely cease. No truly interbedded lava or 
volcanic tuff has been found in either the Millstone Grit or the Coal-measures. 
Yet both these subdivisions have been invaded by extensive intrusive sheets. 
The date of the intrusion cannot be satisfactorily settled. It is evident, how- 
ever, that between the volcanic action, represented respectively by the older 
and the later Carboniferous formations, an enormous interval must have 
elapsed. This question will be discussed in a subsequent part of the present 
paper. 
The circumstances under which the older half of the Carboniferous system — 
of central Scotland was accumulated require to be kept in mind when we 
attempt to follow the history of the contemporaneous volcanic phenomena of 
the region. At the beginning of the Carboniferous period, the conditions — 
under which the Old Red Sandstone had been accumulated still in part con-— 
tinued. The great lacustrine basin in which the 20,000 feet of Lower Old Red 
Sandstone had been deposited had been in great measure effaced. But com- 
paratively shallow areas of fresh or brackish water occupied its site. Its” 
conglomerates and sandstones had been uplifted and fractured. Its vast ranges 
of volcanic material, after being deeply buried under sediment, had been once — 
more laid bare, and now extended as ridges of land, separating the pools and 
lagoons which they supplied with sand and silt. We know little as yet of the 
flora which at the close of the long Old Red Sandstone period covered these — 
ridges. It probably closely resembled that which, succeeding it, has been — 
preserved in the sandstones and shales of the upper group of the Calciferous 
sandstones. Of the fishes, however, which frequented the waters, some 
knowledge has been gathered from the well-known sandstone of Dura Den. 
With many characteristic changes and differences, these fishes retain much of 
the peculiar type of the older divisions of the Old Red Sandstone. Though 
the strata in which they lie pass insensibly into and are intimately bound up— 
with the overlying Carboniferous beds, the fossils themselves have an unmis-_ 
takable Old Red Sandstone facies. ¢ 
The red sandstones at the base of the Carboniferous system are almost — 
everywhere unfossiliferous. Beyond Cockburnspath they have yielded a few 
scales of Holoptychius and other forms like those of Dura Den. Elsewhere 
their barren monotonous character contrasts them with the dark shales and | 
white sandstones of the overlying group of rocks. That they were laid down | 
on a very uneven floor is shown by the way in which they are overlapped by 
