352 PROFESSOR GEIKIE ON THE 
subterranean movements. They show how extensive and prolonged was the 
activity of the volcanoes which over these regions must have dotted the surface 
of the Silurian sea. 
Thus, in spite of the prevalent and long-continued subsidence, the succession 
of Silurian deposits over the British area was frequently interrupted. Pro- 
fessor RAMSAY many years ago drew attention to these local breaks in the suc- 
cession of the strata, and connected them with corresponding local interrup- 
tions of the continuity of organic forms.* Reflecting upon this curious and 
varied history, we may believe that apart from any upheaval of the sea-floor, a 
mere cessation of the downward movement could not but produce a great change 
in the geography of the region, and, consequently, upon the distribution of life. 
Let us suppose that such a pause in the subsidence took place at the close of the 
Silurian period. The wide but shallow Silurian sea would come to be silted up 
over considerable tracts. Sand-bars and mud-flats would gradually rise above 
the level even of the highest tides. Portions of the sea would thus be isolated 
from the main body of the water, and under the influence of evaporation would 
be converted into salinas, too bitter for the support at least of an abundant 
fauna. Yet, on the breaking down of any of these barriers by the waves, an — 
irruption of the sea into the lagoons might temporarily reintroduce some of the — 
forms of life which had previously been killed by the concentration of the 
water. If, now, we suppose that in addition to the shallowing and narrowing 
of the sea by means of the constant deposit of sediment, portions of the bottom 
were from time to time ridged up above the sea-level, so as still further to com- 
plete the isolation of different water-basins, and even to permit these to become 
slowly freshened into lakes, we can realise how completely the aspect of the 
north-western European area would thus be changed, and how the Silurian 
fauna might be permanently driven from that region. 
It was by some such physical changes as these that the era of the Old Red 
Sandstone was ushered in. In spite of many subsequent dislocations, of enormous 
denudations, and of the overspread of later sedimentary formations, it is still 
possible to trace some of the more salient features in the geography of Britain 
at that ancient geological epoch, and to follow the more marked vicissitudes 
through which the region passed during the vast interval which separated the 
Silurian from the Carboniferous period. 
To do this with completeness, as far as the evidence permits, will require a 
much more thorough analysis and comparison of the infra-Carboniferous sec- 
tions, both in this country and on the Continent, than has yet been attempted. 
I believe, however, that a much larger and more comprehensive fragment of 
the record of the Old Red Sandstone remains in the northern half of the 
* “Quart. Journ, Geol. Soc.,” vol. xix. (1863) p. xxxvi. 

