Presidential Address to Geological Section. 463 



relations of the Silurian rocks of the south of Scotland by Professor 

 Lapworth. The history of research relating to that tableland, and of all 

 his contributions to the problems 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 geology. 

 The first, on " The Moffat 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 anticlinal 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 

 region, 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 Place in the Upland Sequence," ' he 

 indicated the distribution and variation of the Moffat terrane (Upper 

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

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

 Silurian tableland, its endless overfolds, its endoclinal and exocliual 

 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 comj)lete was the zonal method of mapping 

 adopted by Professor Lapworth, and so accurate were his generalizations, 

 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 recognized 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 graptohtes 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 

 Lap worth'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 palseontological value.* This discovery has enabled 



1 Quart. Journ. Gcol. Soc, vol. xxxiv, p. 240. 



2 Ibid., vol. xxxviii, p. 537. 



3 Geol. Mag., Dec. Ill, Vol. VI (1889), p. 20. 

 * Trans. Roy. Soc. Edinb., vol. xxxix, p. 827. 



