92 SECTIONAL ADDRESSES. 



free from dark mud, but we have still to learn what were the conditions 

 that determined its formation. 



Algal Limestones. 



Mr. E. B. Wethered and the late Prof. H. A. Nicholson were the 

 earliest British workers to pay attention to the calcareous algae and other 

 somewhat obscure organisms which are met with in the Carboniferous 

 Limestone. But it was not till Prof. Garwood turned his attention to the 

 subject that their importance as rock-builders was fully recognised. His 

 presidential address to Section C (Birmingham, 1913) and papers in the 

 Geological Magazine** have led to the recognition of these organisms at 

 several horizons and in many localities. 



In the North-west Province, Garwood describes*^ well-marked algal de- 

 velopments at three levels, viz. : (1) the Solenojpora sub-zone (lower Ci), 

 (2) near the base of the Seminula gregaria sub-zone (lower C.^), and (3) the 

 Girvanella nodular bed (base of D.J which forms the top of the Great Scar 

 limestone. 



The Girvanella band is met with in many of the Yorkshire dale sections, 

 and occurs in the equivalent of the Great Scar limestone of other areas, 

 such as the Melmerby scar limestone of Alston, and the Oxford limestone 

 of North Northumberland. 



Algse also occur at the base of Dj in the Woodend, Eedesdale and Dun 

 limestones of Northumberland. In the Berwick-on-Tweed Memoir 

 Girvanella-Yik.Q nodules are recorded from several levels.*® 



Garwood*'' mentions that Mitcheldeania is found in the highest 

 calcareous bands of the Cement Stones at a number of localities in 

 Northumberland. 



The rhythmic succession in the Yoredale series of Wensleydale, 

 described by Hudson,** ends in each case with algal limestone deposited 

 under shallow-water conditions. His table of succession shows such 

 bands in the Middle, Simonstone, Hardraw Scar, and Gayle limestones. 

 There are also indications of a rhythmic succession in the Northumberland 

 sequence. In West Cumberland algal bands occur at a number of levels. 

 Edmonds*^ mentions three principal ones in addition to the Girvanella 

 band at the base of D.2. 



A study of the rocks of the Avon section^" has shown that a strong 

 algal development occurs at three levels, Km, C.^-Si, and the top of S.^. 

 The well-known ' concretionary beds ' at the top of S.^, and the pisolites 

 which occur at various levels in Sj and S.^, were shown to be — in part, at 

 any rate — algal. Algal limestones have been recognised in all the other 

 Carboniferous Limestone sections in the Bristol district that have been 

 studied in detail wherever rocks deposited under MocZtoZa-phase conditions 

 occur. Thus algal Umestones occur in both Sj and S.^ in the Wickwar 



«« Dec. v., vol. X. (1913), pp. 440, 490, 545, and Dec. vi., vol. i. (1914), p. 265. 



" Q.J.G.S., vol. Ixviii. (1912). 



^^ Hem. Oeol. Surv., Berwick-on-Tweed (1926), pp. 18 and 23. 



" Geology in the Field, p. 676. 



" Proc. Yorks Oeol. Soc, vol. xx., pt. 1 (1923-24), p. 125. 



«» Geol. Mag., vol. lix. (1922), p. 119. 



*» Q.J.G.S., vol. Ixxvii. (1921), and Geol. Mag., vol. Iviii. (1921), p. 546. 



