426 



THE SILURIAN ROCKS 



LEADHILLS DISTRICT. 



As a prelude to the description of the Leadhills district, brief reference 

 must be made to the geological structure of the classic region of Moffat 

 which embraces the type-sections described by Professor Lapworth, in proof 

 of the sequence and relations of the graptolite-shales of the central part of 

 the tableland. The Moffat black shale series forms narrow boat-shaped 

 areas, being brought to the surface along sharp anticlinal folds, in the midst 

 of the younger Tarannon rocks. From the preceding table it will be seen 

 that the members of the Moffat series have been grouped in three divisions : 

 Glenkiln, Hartfell, and Birkhill ; corresponding to the Upper Llandeilo, 

 Caradoc, and Llandovery rocks of Wales respectively, the whole sequence 

 not exceeding 300 feet in thickness. Where denudation has proceeded far 

 enough to expose strata underlying the Glenkiln shales, these consist of 

 Radiolarian cherts and fine tuffs of Lower Llandeilo and Arenig age. The 

 famous Dobb's Linn section at the head of Moffatdale, clearly shows the 

 divisions and structural relations of the Moffat series. When the Moffat 

 series is traced northwards on successive anticlines, towards the northern 

 margin of the central belt of Tarannon rocks, the subdivisions of the Birk- 

 hill shales (Llandovery) disappear and are replaced by coarser sediments. 

 Hence at certain localities it is not easy to trace with precision the northern 

 limit of the Tarannon belt. 



In the districts of Abington and Leadhills there is a remarkable develop- 

 ment of Arenig, Llandeilo, and Caradoc rocks, of greater thickness than 

 their representatives in the Moffat region. A remarkable feature of these 

 northern districts is the occurrence of Arenig volcanic rocks, comprising 

 lavas, agglomerates, and tuffs exposed in the cores of many anticlinal folds. 

 They are immediately overlain by fine mudstones containing abundant 

 hingeless brachiopods (see list), and these in turn by red, dark, and gray 

 cherts charged with Radiolaria (see list). Next in order come the Glenkiln - 

 Hartfell shales yielding their characteristic graptolites of the Moffat region. 

 This sequence is exposed in various folds in the districts of Abington, Lead- 

 hills, and Saiiquhar. Part of the Lower Caradoc rocks of the northern 

 districts is represented by grey wackes, shales, and fossiliferous conglomerates 

 (Duntercleuch, Wallace's Cast, and Kilbucho, see list), while the Upper 

 Caradoc rocks are represented mainly by the Lowther Shales which, to the 

 east of the Clyde basin, include the fossiliferous limestones and conglomerates 

 of Wrae, Glencotho, and Winkston. This change in the type of sedimenta- 

 tion of the Upper Caradoc rocks is also observable in that of the Lower 

 Hartfell and Glenkiln black shales (Lower Caradoc and Upper Llandeilo) 

 when the latter are traced northwards to the margin of the tableland. 

 These in turn are replaced by coarse sediments and the graptolites of the 

 Glenkiln group occur in thin dark seams, interleaved in greywackes and 

 shales. 



FOSSILS FROM ARENIG MUDSTONES, ABINGTON DISTRICT. 



Tetragraptus fruticosus, Hall. 



T. quadribrachiatus, Hall. 



Annelid jaws (?). 



Caryocaris Wrighti, Salt. 



Acrotreta Nicholson!, Dav. 



A. sp. 



Acrothele granulata, Linnr. 



A. sp. 



Lingula, sp. 



Lingulella lepis, Salt. 



L. quebecensis, Bill. 



Lingulella, sp. 



Linnarssonia, sp. 



Obolella like 0. Sabrinae, Callow. 



Linnarssonia (Obolella) sagittalis, 



Salt. 

 L. sp. 



Kutorgina labradorica, ? Bill. 

 K. sp. 



Discinoid shells. 

 INCERTAE SEDIS. 

 Rods, (Sponge?) 



