FresiderU's Address. 383 



other Lower and Middle Arenig forms in the black shales 

 of Bennane Head, near Ballantrae, in Ayrshire, established 

 the Arenig age of what were known as the "Ballantrae 

 Eocks." 1 The subsequent work of the Geological Survey 

 confirmed that discovery, and also proved that Tetragrajptus 

 and an Arenig Phyllopod, Garyocaris Wrighti, occur in fine 

 tuffs intercalated with the lowest lava flows exposed along 

 the Ayrshire coast north of Ballantrae, and which are the 

 lowest rocks of the Southern Uplands.^ It further shows 

 that Tetragraptus occurs, along with Kutorgina^ Ohoklla^ 

 and other hingeless Brachiopods, in mud-stones immediately 

 overlying the volcanic rocks, and beneath the zone of Eadio- 

 larian Chert in the Abington district of Lanarkshire, proving 

 that Arenig rocks are exposed on the crests of several of the 

 more deeply denuded anticlines right across the Southern 

 Uplands.^ 



The rocks of the Eadiolarian Zone already mentioned 

 have not yielded any other remains of life than Eadiolaria, 

 of which they are almost entirely composed, with the excep- 

 tion of a few sponge-spicules,* and are thus entitled to be 

 looked upon as having been true radiolarian oozes accumu- 

 lated in a clear, though not necessarily an abyssmal ocean. 

 As no fossils of value for zoning purposes have been obtained 

 from these rocks, their relative age has to be determined by 

 other means. During the progress of the Geological Survey 

 along the Ayrshire coast south of Ballantrae, a band of dark 

 shale was discovered immediately overlying the Eadiolarian 

 Chert Zone, which yielded an abundant Graptolite fauna of 

 the Ccenographcs gracilis zone, denoting an Upper Llandeilo 

 horizon.'^ It follows, therefore, that the Eadiolarian Chert 

 Zone of the south of Scotland, notwithstanding that it is 

 only about 60 feet thick, must represent at least the de- 

 posits of Upper Arenig and Lower Llandeilo time. 



1 Geol Mag., Jan. 1889, part i., "The Ballantrae Rocks." 

 -Mem. Qeol. Sur., "The Silarian Rocks of Britain," pp. 435, 439, 440, 

 1889. 

 ^IMd., pp. 422, 429. 



"^ Ann. and Mag. Nat. Hist., vol. vi. p. 40. 

 ^ Mem. Geol. Sur., "The Silurian Rocks of Britain," pp. 422, 429, 1900. 



