514 



JVA TURE 



[September 19, 1901 



form of granite ranging along the Grampian chain from Aber- 

 deenshire to Argyllshire. In connection with one of these 

 masses a remarkable paper appeared in 1892 which in my 

 opinion has profoundly influenced petrological inquiry in Scot- 

 land from the light which it threw on the relations of a con- 

 nected series of petrographical types in a plutonic complex. I 

 refer to the paper on the " Plutonic Rocks of (iarabal Hill 

 and Meall Breac," by Mr. Teall and Mr. Dakyns {Quart, 

 jfourn. Gcol. Soc, vol. xlviii. p. 104). 



The authors showed that this plutonic mass comprises granite, 

 tonalite, augite-diorite, picrites, serpentine, and other com- 

 pounds. ^Ir. Teall regards the members of this sequence as 

 products of one original magma by a process of differentiation, 

 the peridotites being the oldest rocks, because the minerals of 

 which they are composed are the first to form in a plutonic 

 magma. As the process of consolidation advances, rocks of a 

 varied composition arise, in the order of increasing acidity, viz., 

 diorites, tonalites, and granites. The most acid rock consists 

 of quartz and orthoclase, which may represent the mother liquor 

 after the other constituents had separated out. Mr. Teall con- 

 cludes that progressive consolidation of one reservoir gives rise 

 to the formation of magmas of increasing acidity, and hence 

 that basic rocks should precede the acid rocks. This theory of 

 magmatic differentiation — so strenuously advocated by Brogger, 

 Vugt, Rosenbusch, Iddings, Teall. and others — was first applied 

 to the interpretation of varied types of plutonic masses in Scot- 

 land by Mr. Teall in the paper referred to. Since then he 

 has extended its application to the granite masses in the 

 Silurian tableland of the south of Scotland, which include rocks, 

 ranging from hyperites at the one end»to granitite with microcHne, 

 and aplite veins at the other.' Many of the phenomena pre- 

 sented by the newer granite masses of the Eastern Highlands 

 seem to lend support to this theory. These views, indeed, have 

 permeated the petrological descriptions of the granitic protru- 

 sions in the counties of Aberdeen and Argyll which have been 

 given by Messrs. Barrow, Hill, Kynaston, and Craig - in recent 

 years. 



One of the remarkable advances in Scottish geology during 

 the period under review is the solution of the order of succession 

 and tectonic relations of the Silurian rocks of the south of Scot- 

 land by Prof. Lapworth. The history of research relating to 

 that tableland, and of all his contributions to the problems con- 

 nected 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" (Quart. Jonrn. Geol. .Soc, 

 vol. xxxiv. p. 240), demonstrated, by means of the vertical dis- 

 tribution of the graptolites, the order of succession in those fine 

 deposits (black shales and niudstones), 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'' {I'hW., vol. xxxviii. p. 537), 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 Scot- 

 land and their Place in the Upland Sequence" (Geol. Mag., 

 December 3, vol. vi. p. 20), he indicated the distribution and 

 variation of the Moffat 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 

 conditions of deposition. Finally he proved that the compli- 

 cated tectonics of the Silurian tableland, its endless 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 Prof. Lapworth, and so accurate were his 

 generalisations, that few modifications have been made in his 



ork. 



1 " Annual Report of the Geological Survey for 1896," p. 40 ; see also 

 "The Silurian Rocks of Scotland" (Geological Survey Memoir, 1899, 

 p. 607). 



- Annual Report of the Geological Survey for 1897," p. 87 ; for i8g8, 

 pp. 25-28 : see aho paper on " Kentallenite and its Relations to other 

 Igneous Rocks in Argyllshire" (Quart. Joum. Gcol. Soc, vol. Ivi. 



NO. 1664, VOL. 64] 



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 de- 

 velopment of which occurs at Ballantrae in Ayrshire, where their 

 igneous character was recognised by Prof Bonney. But they 

 appear in the cores of numerous anticlines over an area of about 

 1500 square miles, forming one of the most extensive volcanic 

 areas of Pateozoic 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 2000 square 

 miles. The deposition of the Radiolarian ooze must have oc- 

 cupied 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 Prof. Lapworth's researches bearing on thegraptolitic deposits 

 of the Upper Llandeilo period, which must have been laid 

 down on the sea-floor near the limit of the land-derived sedi-' 

 ment. 



Of special interest is the new fish fauna found by the Geo- 

 logical 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 palteonto- 

 logical value (Trans. Roy. Soc. Edin., vol. xxxix. p. 827). 

 This dii-covery 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 Pteraspidae alone. 

 He has further shown that the Coclolepidae were not Cestraciont 

 sharks to which the Onchiis spines belonged, but Heterostraci, 

 though probably of Elasmobranch origin, judging from the 

 shagreen-like scales. The Coclolepidae are common fishes in the 

 Ludlow and Downtonian rocks of Lanarkshire. The genus, 

 Thelodits, 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 Upper Silurian rocks of 

 Oesel, is here represented for the first time by nearly complete 

 forms. But it is remarkable that no Onchiis spines, nor any 

 Pteraspidae, nor Ccplialaspidae have been found in the Lanark- 

 shire strata, the nearest related genus to Cephalaspis being 

 Ateleaspis, which, however, represents a distinct family. 



The group of sandstones, conglomerates, shales, and mud- 

 stones that form the passage-beds between the Ludlow rocks 

 and the Lower Old Red Sandstone in Lanarkshire are now re- 

 garded as the equivalents of the Downtonian strata in Shrop- 

 shire, and are linked with the Silurian system. The mudstones 

 of this group, containing the new fish fauna, likewise yield 

 ostracods, phyllocarid crustaceans, and eurypterids — forms which 

 connect these beds with the underlying Ludlow rocks. The 

 band of greywacke-conglomerate, that extends from the Pent- 

 land Hills into Ayrshire, composed largely of pebbles derived 

 from the Silurian tableland, is now taken as the base line ot the 

 Lower Old Red Sandstone on the south side of the great midland 

 valley of Scotland. 



The period under review has been marked by important ad- 

 ditions to our knowledge of the Old Red Sandstone formation. 

 In 1S78 appeared a valuable monograph by Sir Archibald 

 Geikie on " The Old Red Sandstone of Western Europe " 

 (Trans. Roy. Soc. Edin., vol. xxviii. p. 345), by far the most 

 important treatise on this subject since the publication of Hugh 

 Miller's classic work published in 1S41. Following upthe view 

 maintained by Fleming, Godwin-Austen, and Ramsay, that the 

 deposits of this formation were laid down in lakes or inland seas, 

 he defined the geographical areas of the various basins in the 

 British area, giving to each a local name. He gave an outline 

 of the development of the rocks north of the Grampians, in 

 Caithness, Orkney, and Shetland. He advanced an ingenious 

 argument in favour of correlating the Caithness flagstone 

 series (middle division, Murchison) with the Lower Old 

 Red Sandstone south of the Grampians. He contended 

 that "the admitted pateontological distinctions between 

 the two areas are probably not greater than the striking litho- 

 logical differences between the strata would account for, or 

 than the contrast between the ichthyic faunas of adjacent but 

 disconnected water basins at the present time." Sir A. Geikie 



