TRANSACTIONS OF SECTION C. 625 



connected with it, has been given in detail in the recent volume of the Geological 

 Survey on that formation. At present it will be sufficient to refer to his three 

 classic papers, which, in my opinion, record one of the great achievements in 

 British pfeology. The first, on ' The Moftat Series,' ' demonstrated, by means of 

 the vertical distribution of the graptolites, the order of succession in those fine 

 deposits (black shales and mudstones), which were laid down near the verge of 

 sedimentation, and are now exposed in auticlinnl folds in the central belt. The 

 second, on 'The Girvan Succession,'- showed how certain graptolite zones of the 

 Moffat shales are interleaved, in the Girvan rejrion, with conglomerates, grits, 

 sandstones, flagstones, mudstones, shales, and limestones, charged with all the 

 varied forms of life found in shallow seas or near shore. In the third, on ' The 

 Ballantrae Rocks of the South of Scotland and their I'lace in the Upland 

 Sequence,' ^ he indicated tlie distribution and variation of the ^loffat terrane 

 (Upper Llandeilo to Upper Llandovery) and of the Gala terrane (Tarannon), 

 which form the greater part of the uplands. He further pointed out how the 

 rocks and the fossils vary across the uplands according to the condit'ons of 

 deposition. Finally he proved that the complicated tectonics of the Silurian 

 tableland, its endle.'^s overfolds, its endoclinal and exoclinal structures, can be 

 unravelled by means of the graptolite zones. These researches disposed of the 

 order of succession based on Barrande's doctrine of Colonies, and established the 

 zonal value of graptolites as an index of stratigraphical horizons. So complete 

 was the zonal method of mapping adopted by Professor Lapworth, and so accurate 

 were his generalisations, that few modifications have been made in his work. 



In the course of the re- examination of the Silurian tableland by the Geological 

 Survey some important additions were made to our knowledge of the Silurian 

 system as there developed. Underlying all the sediments of the uplands there is 

 a series of volcanic and plutonic rocks of Arenig age, the largest development of 

 which occurs at Ballantrae in Ayrshire, where their igneous character was recog- 

 nised by Professor Bonney. But they appear in the cores of numerous anticlines 

 over an area of about 1,500 square miles, forming one of the most extensive 

 volcanic areas of Palaeozoic age in the British Isles. These volcanic rocks are 

 overlain by a band of cherts and mudstones, succeeded by black shales yielding 

 Glenkiln graptolites of Upper Llandeilo age. The cherts, which are abundantly 

 charged with Radiolaria, implying oceanic conditions of deposition, are about 

 70 feet thick, and have been traced over an area of about 2,000 square miles. 

 The deposition of the Radiolarian ooze must have occupied a long lapse of time. 

 Indeed the cherts and mudstones represent the strata which, in other regions, form 

 the Upper Arenig and Lower Llandeilo divisions of the Silurian system. They 

 furnish interesting evidence of the oceanic conditions which here prevailed in 

 early Silurian time, and form a natural sequel to Professor Lapworth's researches 

 bearing on the graptolitic deposits of the Upper Llandeilo period, which must have 

 been laid down on the sea-floor near the limit of the land-derived sediment. 



Of special interest is the new fish fauna found by the Geological Survey in the 

 Ludlow and Downtonian rocks between Lesmahagow and Muirkirk, which the 

 researches of Dr. Traquair have shown to be of great biological and palaeonto- 

 logical value.'* This discovery has enabled him to give a new classification of the 

 Ostracodermi, to enlarge the order of the Heterostraci, which now includes four 

 families, instead of the Pteraspid<e alone. He has further shown that the 

 Coslolejpidcel-weTe not Cestraciont sharks to which the Onchus spines belonged, but 

 Heterostraci, though probably of Elasmobranch origin, judging from the shagreen- 

 like scales. The Coelolepida are common fishes in the Ludlow and Downtonian 

 rocks of Lanarkshire. The genus, Thelodns, first described by Agassiz from 

 detached scales in the Ludlow bone-bed, and subsequently figured and described 

 by Pander and Rohon from scales in the LTpper Silurian rocks of Oesel, is here 

 represented for the first time by nearly complete forms. But it is remarkable that 

 no Onchus spines, nor any Pteraspidee, nor Cephalaspid<e. have been found in the 



' Quart. Journ. Geol. Soc, vol. xxxiv. p. 240. " Ibid., vol. xxxviii. p. 537. 



* Geol. Mag., Dec. 3, vol. vi. p. 20. ■• Trans. Roy, Soc. Edin., vol. xxxix. p. 827. 



