CARBONIFEROUS LIMESTONE. I55 



thickness, but often massive, sonae of which are oolitic in structure. 

 It is frequently traversed by veins of calc-spar, and by strings or 

 nodules and bands of chert. Layers of chert are sometimes met 

 with which merge gradually into the masses of limestone, and 

 the rock is locally much stained by infiltrations of iron-ore. It 

 has long been popularly termed the Mountain Limestone, and it 

 was spoken of by Sedgwick as the Great Scar Limestone.^ 



The beds are fossiliferous, and chiefly of organic origin, being 

 often largely made up of fragments of Brachiopoda, Corals, Polyzoa, 

 Crinoids, and Foraminifera ; ^ but sometimes few traces of organic 

 remains are discernible, without the aid of a microscope. 



In thickness the Carboniferous Limestone is estimated to attain 

 3000 feet. In many localities it is much less, and this variation is 

 caused to some extent by the unequal surface of the older rocks on 

 which the Limestone sometimes rests. ^ 



The Corals include Lithostrotion [striatuni) hasaltiforme, Lithoden- 

 droji irregularcy Syn'ngoponi reticulata, Lonsdalcia florifonnis, 

 Zaphrentis cylindj-ica, Chcctetes radians, ClissiophylJum, and Cyatho- 

 phyllum regium} 



The Polyzoa include Fenestella, Ceriopora, and Polypora ; the 

 Crinoids include Cyathocriiitis planus, Actinocrinus polydactylus, 

 Platycrinus coronatus, etc. The rock is often an Encrinitic or 

 Crinoidal limestone. The name of St. Cuthbert's Beads has been 

 applied to the loose joints of Encrinites. 



The Brachiopoda include Productus sejuireticulatus, P. piinctatus, 

 P. giganteus, Spirifera cuspidata, S. ^lineata, St>vphomena analoga, 

 Orthis resupinata, O. Michelifii, Rctzia radialis, Chondes Hardrensis, 

 C. sordida, Streptorhynchus cre7iistria, Athyris Royssii, Rhynchondla 

 pugtius, R. pleurodoti, etc. 



The other forms of Mollusca include Posidonomya Becheri, 

 Avicidopedeii papyraceus, A. granosus, Conocardiiwi gigantnim, 

 Conularia quadrisulcata, Plciirotomaria biserrata, Loxoneina, Euoin- 

 phalus nodosus, Bdlerophon apertus, Orthoceras cinctum, Goniatites 

 Listeri, etc. ; and the Trilobites include Phillipsia, GriJJithides, and 

 Brack J '711 dop us J' 



Among the Fishes are Ctenacanthus, Psammodus, Cochliodus, 

 Lophodus, Cladodus, Pdalodus, and Folyrhizodus.^ 



Fossils are best obtained from old weathered surfaces and joints 

 of the limestone, where Nature has left them standing out in relief 

 from the more easily dissolved matrix. 



The name Scar Limestone, which was applied by Sedgwick to 



1 T. G. S. (2), iv. 70. 



2 H. C. Sorby, Address to Geol. Soc. 1879; E. Wilson, Midland Naturalist, 

 iii. 220. 



3 Dr. C. Ricketts, G. Mag. 1883, p. 350. 



^ See H. A. Nicholson, On the Structure and Affinities of the Tabulate Corals of 

 the Palaeozoic Period, 1879. 



5 Dr. H. Woodward, G. Mag. 1883, p. 534; 1884, p. 484. 



« See J. W. Davis, Trans. R. Dublin Soc. ser. 2, vol. i. p. 327, 18S3 ; R. H. 

 Traquair, Carb. Fishes (Palseontogr. Soc). 



