President's Address. 385 



border be correct, then these rocks will have a considerable 

 influence in fixing the date of the contortion of the Highland 

 rocks along the Highland border, just as the evidence obtained 

 from the Durness Limestone will have to be considered when 

 dealing with the date of the ridging up and alteration of the 

 rocks in the north-west Highlands. From the nature and 

 distribution of the Durness Limestone and of its fossil con- 

 tents, it is almost certain that a clear ocean, free from land- 

 borne sediment, existed over what are now the north-west 

 Highlands in late Cambrian and early Silurian (Arenig) 

 time, and if the Highland border rocks are the continuation 

 of those of Ballantrae, then a clear sea, away from land, must 

 have stretched in Arenig times from what are now the 

 Southern L'plands to the Highland border at least, since the 

 Eadiolarian Cherts extend over all this area. It is certain 

 that the Durness Limestone is involved with the other rocks 

 of the north-west Highlands in the great movements which 

 have brought about the structure of that region, and in like 

 manner the supposed Ballantrae rocks partake in the extreme 

 folding to which the southern Highland rocks have been 

 subjected; and, as far as Perthshire is concerned, they are in 

 the same state of metamorphism as the adjoining Highland 

 schists. 



Eor our purpose it is necessary to return to the considera- 

 tion of the newer Silurian rocks of the Southern Uplands. 

 In 1878 Professor Lapworth's epoch-making memoir on the 

 " Moffat Series " made its appearance.^ In it he proved that 

 the three great divisions of " Siluria " are represented in a 

 condensed form in about 300 feet of strata in the Moffat area. 

 Each division is characterised by a distinct Graptolite Fauna ; 

 the Llandeilo by his " Grlenkiln," the Caradoc by his " Hart- 

 fell," and the Llandovery by his ''Birkhill" Group of Grap- 

 tolites. He further demonstrated that each division is 

 capable of being subdivided into zones characterised by one 

 or more zonal forms of Graptolite. Having placed these 

 facts on an unassailable basis, he made use of the knowledge 

 thus gained to work out the structure of the Girvan area, 



^ Quart. Jour. Geol. Soc, vol. xxxiv. p. 240, 1878. 



