F7'esidenfs Address. 387 



land appears throughout Silurian times to have been per- 

 sistently towards the north, and the open sea towards the 

 south. Though the detailed structure of the area over which 

 the rocks holding the Graptolites of the Stockdale Shales in 

 Scotland has still to be made out, the Tarrannon age of the 

 rocks distributed over a vast tract of country, stretching from 

 St Abb's Head to the Mull of Galloway, is made certain. 



The correlation of the Wenlock Ludlow rocks in Scotland, 

 with their equivalents in England and the rest of Europe, 

 had been accomplished prior to 1880. During the revision 

 of the Upper Silurian rocks in the years 1896, 1897, in the 

 Lesmahagow district of Lanarkshire, Messrs Macconochie 

 and Tait ^ obtained a large suite of Fish remains from rocks 

 which up to that time had been considered as the basement 

 members of the Lower Old Eed Sandstone. The Fish Fauna 

 has been shown by Dr Traquair to be very nearly alHed to 

 that of the Ludlow " Bone-Bed " of the Welsh border, though 

 differing from it in many respects, and especially in the fact 

 that the Fish are represented in the Bone -Bed by rolled 

 debris only, while in the Lesmahagow bed they are almost 

 entire. This find has led to the correlation of the beds in 

 which they occur with the Downtonian rocks of the Welsh 

 border, and thus forming the highest subdivision of the 

 Silurian system. The index map accompanying volume i. of 

 the Memoirs of the Geological Survey of the United Kingdom 

 shows the areas in which the rocks have been transferred 

 from the Lower Old Red Sandstone to the Silurian forma- 

 tion.2 



Before quitting the subject of the Silurian rocks, we may 

 indicate how fossil evidence can be used to fix the date 

 of an unconformability, and show how a marked break 

 took place within the period of the life of the Glenkiln 

 Graptolite Fauna in the Ballantrae region of Ayrshire. South 

 of Ballantrae, as has been above mentioned (p. 383), a dark 

 shale occurs conformably overlying the Radiolarian Cherts, 



1 Mem. Geol. Sur., "Summary of Progress for 1897," pp. 72-74, 1898. 

 - Mem. Geol. Sur., "The Silurian Rocks of Britain," vol. i, (index map), 

 1900. 



