498 Prof. Garwood — Calcareous Algce. 



Some of the best examples known to me occur associated with. 

 Ortonella in the ' Productus glohoms band ' near the summit of the 

 * Athyris glahridria zone ' in the 8hap district. They occur here in 

 considerable masses, often many inches in thickness, and form 

 undulating layers parallel to the bedding, and somewhat resembling 

 luuge ripple-marks. In all cases they appear to be due to the 

 precipitation of carbonate of lime in the neighbourhood of algal 

 growths. I have also met with similar deposits, not only at other 

 horizons in the Lower Carboniferous rocks of the North of England, 

 but also in the Forest of Dean and in the rocks of the Avon Gorge ; 

 while quite recentlj^ Mr. C. H. Cunnington has sent me examples 

 from several horizons in the Carboniferous Limestone of South Wales. 



Girvanella. — This organism appears to play a considerable part in 

 the formation of calcareous deposits in the Lower Carboniferous 

 rocks of Britain. Its presence in these rocks was first suggested 

 by the late Professor Nicholson,^ who wrote: "I have found 

 some of the Carboniferous Limestone of the North of England to 

 contain largely an ill-preserved organism, which will, I think, prove 

 to be referable to Girvanella.''^ This prophecy has turned out to be 

 fully justitied, not only as regards the North of England, but 

 also in the case of the Lower Carboniferous beds of other districts. 

 In 1890 Mr. E. Wethered described' two new forms of this genus 

 from the Lower Carboniferous rocks of the Avon Gorge and Tort- 

 worth, viz. G. incrustans, with tubes having a diameter of 0*1 mm., 

 and G. Bucii^ with a diameter of •O'imm. Mr. Wethered appears 

 to rely chiefly on the size of the tubes for the differentiation 

 of these species, but as this distinction was made at the time 

 when Girvanella was still considered to belong to the Rhizopods, 

 and as the size of the tubes frequently varies in the same 

 specimen, it is doubtful whether these species can be maintained. 

 Mr. Wethered's specimens were obtained from the limestone near 

 where the Bridge Valley Road joins the river bank, apparently at the 

 base of Dr. Vaughan's tipper Dibunophyllum Zone. The position of 

 this limestone is of interest, as it appears to correspond very closely 

 with the horizon of the '(rJ/Tflw^/^rt Nodular Bed', which forms a well- 

 marked band at the base of the Upper Dibunophyllum Zone throughout 

 the whole of the North and North- West of England. Indeed, I have 

 traced this band at intervals from the neighbourhood of Ford, near 

 the Scottish Border, southwards through Northumberland and the 

 Pennine area to Penygent, and from the west coast at Humphrey 

 Head through Arnside and Shap to the east coast near Dunstanburgh. 

 These organisms must, therefore, have flourished at this period over 

 an area of at least 3,000 square miles in the North of England alone. 



The best exposure showing the important development of these 

 Girvanella nodules is to be found on the dip slopes forming the eastern 

 shore of Humphrey Head in Morecambe 13ay, where the base of the 

 Upper Dibunophyllum Zone is exposed over a considerable area. 



1 Geol. Mag., Dec. Ill, Vol. V, 1888. 



" Q.J.G.S., vol. xlvii, p. 280, pi. xi, figs. 1, 2, 1890. 



(To he concluded in the December Number.) 



