President's Address. 7 



counterpart of the strata exposed along the axis of older 

 Palaeozoic rocks, stretching from Canada through the eastern 

 States of North America. In the latter region, the Silurian 

 strata of Sutherlandshire are represented by — (1.) the Pots- 

 dam Sandstone, always described as being vertically piped by 

 Scolithiis, like the " pipe-rock ; " (2.) the Calciferous group ; 

 and (3.) part of the Trenton Limestone. Now, it might be 

 contended that this remarkable identity in the order of suc- 

 cession of the beds and fossil contents in these two widely- 

 separated areas points to contemporaneous deposition ; and 

 on the other hand it might be argued with greater probability 

 that the beds are homotaxial, or in other words that the same 

 relative succession of events prevailed in the two areas, 

 though not at the same time. Whichever view is adopted, I 

 think that there can be little doubt that some old shore-line or 

 shallow sea must have stretched across the North Atlantic or 

 Arctic Oceans, along wdiich the forms migrated from one 

 province to the other, and that some barrier must have cut 

 off this area from that of Wales and Central Europe. 



Another excellent illustration of the value of fossiliferous 

 zones as an aid to the solution of the geological structure of 

 a special region may be found in the Lower Silurian rocks of 

 the Moffat district, in the south of Scotland. Throughout 

 the Southern Uplands the lithological character of the Lower 

 Silurian rocks are remarkably uniform, consisting mainly of 

 greenish grey wackes, grits, and shales, thrown into a series 

 of parallel folds. The magnificent coast sections at St Abb's 

 Head and in the south-west of Scotland show how the same 

 beds are repeated by normal and inverted anticlines and 

 synclines over wide areas. The difficulty of unravelling the 

 geological structure of the southern uplands is further in- 

 creased by the limited number of fossils obtained from the 

 grits, grey wackes, and shales. There are, however, certain 

 bands of black shales charged with graptoUtes which have 

 been of great service to the field geologist. The exhaustive 

 researches of Professor Lap worth in the Moffat area have 

 conclusively proved that, so far as that region is concerned, 

 the black shales occur along the crests of anticlinal folds, the 

 overlying grits and shales having been removed by denuda- 



