CHAP. XVI 
THE OLD RED SANDSTONE OE BRITAIN 
•.67 
specially concerns us in the present inquiry, in its volcanic records. 
Wherever its true base can be seen, this series passes down conformably 
into Upper Silurian strata. It sometimes reaches a thickness of lo,000 
and even 20,000 feet. There is generally a marked lireak between its 
highest visible strata and all younger formations. Even the upper 
division of the Old Eed Sandstone rests unconformably upon the 
lower.’ Such a hiatus undoubtedly points to a considerable lapse ot 
geological time, and to the advent of important geographical changes 
that considerably modified the remarkable topography of the older part 
of the period. 
Tlie younger division or Upper Old lied Sandstone passes upward con- 
formably into the base of the Carboniferous system. Its red and yellow 
sandstones, conglomerates and breccias, covering mucb more restricted areas, 
and attaining a much less thickness than those of the lower division, 
indicate the diminution and gradual effacemeut ol the lakes of the oldei 
time, and the eventual return of the sea to the tracts from which it had 
heen so long excluded. So vast an interval elapsed between the time 
recorded in the deposits respectively of the two sections ot the Old Eed 
Sandstone that the characteristic forms of aniniid life in the earlier ages 
had entirely passed away, and their places had been taken by other types 
when the, diminished lake-basins of the second period began to be filled up. 
Volcanic action also dwindled to such a degree that in contrast to the 
abundant vents of the older period, only one or two widely scattered groups 
of vents are known to liave existed in the area of the British Isles during 
the later period, and these, after a feelile activity, gave way to a prolonged 
volcanic quiescence, which lasted until the earlier ages of the succeeding or 
Carboniferous period. 
Although geologists are in the habit of grouping the Old Eed Sandstone 
and the Devonian rocks as equivalent or homotaxial lormations, deposited in 
distinct areas under considerably different conditions ot sedimentation, the 
attempt to follow out the sequence of strata in Devonshire, and to trace ^some 
analogy between tlie Devonian succession anil that of the Old Eed Sand- 
stone, presents many difficulties for which no obvious solution suggests itself. 
Into these iiroblems it is not needful to enter further than was done in the 
last chapter. AVe may assume that not improbalily some ot the eruptions 
now to be described were coeval with those of Devonian time in the south- 
west, of England, though we may hesitate to decide which ol them should be 
brought into parallelism. z-m 1 t> i 
As we trace the shore-lines of the ancient basins of the Lower Old Eed 
Sandstone, and walk over the shingle of their beaches, or as we examine the 
silt of their deeper gulfs, and exhume the remains of the plants that shaded 
their borders, and of the fishes that swarmed in their waters, we gradually 
learn that although the sediments which accumulated in some of these basins 
1 QiiaTt. Jouru. Gcal. Soc. vol. xvi. {I860), p. 312. In Wales no break lias actually been dis- 
covered between tlie two divisions of tlie Old Red Sandstone, tliongli it is .suspected to exist 
there also. 
