CHAP. XIX 
LAKE CALEDONIA 
395 
south-east side another similar series of faults, which there skirts the edge 
of the Silurian table-land, has nearly the same effect in precisely defining 
the margin of the Old lied Sandstone. As thus limited, the tract has a 
breadth of about 50 miles in Scotland, while the portion of it now visible 
in the British Isles has an extreme length of about 280 miles (Map III.). 
But though the boundary-faults determine, on the whole, the present 
limits of the tract of Old Bed Sandstone, they do not necessarily indicate 
the shore-lines of the sheet of water in which that great series of deposits 
was laid down. They point to an enormous subsidence of the tract between 
them — a prolonged and extensive sagging of the strip of country that stretches 
across the Midland Valley of Scotland into the north of Ireland. This 
downward movement began as far back as the close of the Silurian period, 
but the marginal fractures and the disruption and plication of the thick 
masses of sandstone and conglomerate which were accumulated 111 the lake 
chiefly took place after the close of the period of the Lower Old Bed Sand- 
stone. I think w^e may reasonably connect these movements with the general 
sinking of the area consequent upon the enormous outpouring of volcanic 
materials during that period. 
Along both the northern and southern margins of the basin there 
occur, on°the farther side of the boundary faults, outlying patches of Lower 
Old Bed Sandstone that rest unconformably on the rocks forming the 
flanks of the hills. These areas possess a peculiar interest, inasmuch as 
they reveal some parts of the shore-line of the lake, and show the relation 
between the earlier rocks and the sediments of the Old Bed Sandstone. 
We learn from them that the shore-line was indented with wide bays, but 
nevertheless ran in a general north-easterly direction. It thus corresponded 
in trend with the present Midland Valley, with the axes of plication among 
the schists of the Highlands as well as among the Silurian rocks of the 
Southern Uplands, and with the subsequent faulting and folding of the 
Old Bed Sandstone, 
I may remark in passing that the conglomerates and other associated 
I 
a 
a, slates an.! pl.yllites ; 6, volcanic conglomerates ; c, anacsite-lava. 
^ In some of the dislocations aloi 
upon itself, and tlie older schists are 
from the Highland area. 
