CHAPTER XVI 
THE VOLCANOES OF THE OLD RED SANDSTONE 
Geological Eevolntions at the close of the Silurian Period— Physical Geo^aphy of the 
Old Red Sandstone— Ohl Lake-basins, their Flora and Fauna— Abundance of 
Volcanoes — History of Investigation in the Subject. 
We now enter npon tlie consideration of tlie records of a notable era in the 
<reoloo'ical evolution of north-western Europe. Up to the close of the 
Silurhin period the long history embodied in the rocks presents a constant 
succession of slowly sinking sea-floors. Wide tracts of ocean stretched over 
most of Europe, and across the shifting bottom, sand and mud, washed Rom 
lands that have long vanished, spread in an ever-accumulating pile. . ow 
and then some terrestrial movement of more than usual potency upraised 
this monotonous sea-bed, but the old conditions of ceaseless waste con- 
tinued, and fresh sheets of detritus were thrown down upon the hroken-np 
heaps of older sediment. All through the vast cycles of time denoted by 
these accumulations of strata, generations of sea-creatures came and went in 
lomr procession, leaving their relics amidst the ooze of the bottom, (mnera 
amf families, once abundant, gradually died out, and gave place to others, the 
onward march of life being slow but uninterrupted. Of the land of the 
time or of the plants and animals that lived on its surface, hardly anything 
is known. The chronicles tliat have come down to us are almost wholly 
records of the vicissitudes of the ocean-bed. i. 0-1 
Over the centre and south of Europe, the marine conditions ol biUirian 
time were prolonged, as we have seen, into the next period, when the 
Uevonian formations were deposited. In that wide region, no marked break 
has been traced between either the sedimentation or the animal life of the 
Silurian and Devonian periods. But in the north-west of Europe a .striking- 
departure took place from the protracted monotony of marine conditions. 
By a series of terrestrial movements that affected the aiea lying to t le 
north of the line of the Bristol Channel, and extended not only to the 
furthest limit of the British Isles, but probably as far as Norway, and 
perhaps even into northern Eussia, the previous widespread conditions of 
marine sedimentation were entirely altered. Instead of the fine oceanic 
silts and sands with their abundant organic remains, and the thick lime- 
