ANNIVEESARY ADDRESS OF THE PRESIDENT. 67 



myself for the present with a more cursory reference to other 

 districts.^ 



Throughout most o£ the area of ' Lake Caledonia ' the present 

 limits of the Lower Old Red Sandstone are sharply defined by large 

 parallel faults. On the north-west side one, or rather a parallel 

 series, of these runs from Stonehaven along the flank of the High- 

 land mountains to the Clyde, thus traversing the whole breadth of 

 the island. On the south-east side another similar series of faults, 

 which there skirts the edge of the Silurian table-land, has nearly 

 the same efi'ect in precisely marking off the margin of the Old 

 Red Sandstone. As thus limited, the tract has a breadth of 

 about fifty miles in Scotland, while the portion of it now visible 

 in the British Isles has an extreme length of about 280 miles. 



But though the boundary-faults determine, on the whole, the 

 present limits of the tract of Old Red 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 Yalley of Scotland into the north of Ireland.^ This down- 

 ward 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 accu- 

 mulated in the lake chiefly took place after the close of the period 

 of the Lower Old Red Sandstone. I think we may reasonably con- 

 nect these movements with the general sinking of the area conse- 



^ My own investigations of this region have been contiaued over an interval 

 of forty years. Besides personally traversing every portion of it, I have mapped 

 in detail, for the Geological Survey, many hundreds of square miles of its area 

 from the outskirts of Edinburgh south-westwards into Lanarkshire, in Ayrshire, 

 and in the counties of Fife, Perth, and Kinross, The Geological Survey maps 

 of the volcanic tracts of the Sidlaw Hills have been prepared by my brother, 

 Prof. James Geikie, and Messrs. H. M. Skae and D. R, Irvine. The Western 

 Ochils were mapped chiefly by Mr, B. N. Peach, partly by Prof, J. Young, Mr. 

 El. L. Jack, and myself ; the Eastern Ochils were surveyed mainly by Mr. H. 

 H. Howell ; while the volcanic belt between the tracts mapped by me in Lanark- 

 shire and in Ayrshire was chiefly traced out by Mr. Peach. As a rule, each of 

 these geologists has described in the Survey Memoirs the portions of country 

 surveyed by him. 



^ In some of the dislocations along the Highland border, the Old Red Sand- 

 stone is bent back upon itself, and the older schists are thus made to recline 

 upon it, as if there had been a push over from the Highland area. 



