CHAP. XIX 
LAKE CALEDONIA 
297 
remarkable volcanic history was enacted of which I now proceed to give 
some account. • 
The Lower Old Red Sandstone of Central Scotland may be conveniently 
divided into three great groups, each of wliich marks a distinct epoch m 
the liistory of the basin wherein they were successively accumulated, 
lowest of these groups indicates a time of quiet sedimentatioii during which 
the basin was defined by plication of the terrestrial crust, ami when by the 
same subterranean movements, some parts of the floor of the lake were 
pushed upward above water, and were then denuded and buried._ ihe 
middle group consists largely of volcanic rocks. It points to the existence 
of lines of active volcanic cones situated along the length ot the lake. 11 e 
uppermost group records the extinction of volcanic action and the gradual 
obliteration of the lake, partly by the pouring of sediment into it, ami 
partly no doubt by the continued terrestrial movements which had origina > 
produced the basin. _ j.- 
It is evident from these records that though volcanic activity continue 
vigorous for a vast period of time, it had entirely ceased in Lake a e onia 
long before the last sediments of the Lower (.)ld Red Sandstone were 
laid down The great cones of the Ochil Hills, for example, sank below 
the waters of the lake in which they had long been a conspicuous 
feature and so protracted was tbe subsidence of the lake-bottom that 
site of these volcanoes was buried under 8000 or 9000 feet of sandstones 
and conglomerates, among which no trace of any volcanic eruptions has 
yet been found. The sagging of the terrestrial crust over an area froni 
which such an enornions amount of volcanic products had lieeii disclimged 
would doubtless lie a protracted process. Long alter the subsidence ot le 
lake-bottom and tlie accumulation of its thick mass of sediments after even 
the entire effacement of the topography and tlie deposition ot the tine v 
Carboniferous formations over its site, the downward movemeiit showed 
itself in the production of gigantic north-east faults, and the sinking 
of the Carboniferous rocks for several thousand feet. These dislocations, 
as was natural, have run through the heart of some of the volcanic groups 
carryiim much of the evidence of the ancient volcanoes out of sight, and 
leaving us only fragnients from which to piece together the records ot a 
volcanic period which is by no means the least interesting in the geological 
for the present to the records of the middle or 
volcanic groiqi, we find evidence of a number of dktinct clusters of vo canoes 
ranged along the whole length of tlie basin The independence ot tl ese 
volcanic districts may be inferred from the tollowing facts 1st, The actual 
vents of discharge may in some cases be recognized ; 2nd, Even where these 
vents have been buried, we may often observe, as we approach their probable 
sites, a marked increase in the thickness of the volcanic accumulations, as 
well as a great development of agglomerates and tuffs; drd. Traced in 
opposite directions, the volcanic materials are found to thin away 01 even 
to disappear. Those from one centre of discharge may be obseived 
