08 PEOCEEDINGS OF THE GEOLOGICAL SOCIETr. 



quent 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 Eed Sandstone that rest uncon form ably on the rocks 

 forming the flanks of the hills. These areas possess a peculiar inte- 

 rest, inasmuch as they reveal some parts of the shore-line of the lake, 

 and show the relation between the older rocks and the sediments of 

 the Old Red 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 

 E-ed Sandstone. I may remark in passing that the conglomerates 

 and other associated materials which have been preserved in these 

 bays and hollows beyond the lines of the great faults, though they 

 lie unconformably on the rocks beneath, are not the basement 

 portions of the Old Eed Sandstone. On the contrary, where their 

 probable stratigraphical horizon can be recognized or inferred, they 

 are found to belong to parts of the series considerably above the base 

 of the whole. They point to the gradual sinking of the basin and 

 the creeping of the waters with their littoral shingles farther and 

 farther up the slopes of the hills on either side. But this is not all 

 the evidence that can be adduced to show that the limits of the lake 

 extended considerably beyond the lines of dislocation between which 

 the present area of Old Red Sandstone mainly lies, ^o one can 

 look at the noble escarpments of the Braes of Doune on the one 

 side, or walk over the upturned conglomerates and porphyrites which 

 flank the Lanarkshire uplands on the other, without being convinced 

 that if the efi*ects of the boundary-faults could be undone, so as to 

 restore these rocks to their original positions, their prolongations, now 

 removed by denudation, would be found sweeping far into the 

 Highlands on the north and into the Silurian uplands on the 

 south. 



If the area of Lake Caledonia ' were taken to be defined by the 

 boundary-faults, it covered a space of about 10,000 square miles. 

 But, as we know that it certainly stretched beyond the limits marked 

 by these faults, it must have been of still greater extent. We shall 

 probably not exaggerate if we regard it as somewhat larger than 

 the present Lake Erie, the superficies of which is about 9900 square 



