Vol. 68.] SUCCESSION IN THE NOKTH-WEST OP ENGLAND. 551 



the I^orfch of England. Other interesting forms also occur wliit;h 

 cannot be fully described here, although they contribute largely to 

 the formation of the Algal Layer at the base of the Seminula- 

 gregaria Sub-zone. With the exception of calcareous algae, plant- 

 remains are but sparingly represented ; mention, however, may be 

 made of ArcJiceosigiUaria vanuxemi^ which was first described in 

 Britain from the Shap District. So far as is known, this species 

 is limited to the lowest zone, in which, however, it occurs at 

 several horizons ; associated with it, occasionally, are specimens 

 of Bothrodendron.^ 



Siliceous organisms are on the whole rare, though some of the 

 spherical bodies associated with GalcispJierce may perhaps come 

 under the category of radiolaria, the siliceous skeleton of which has 

 been subsequently replaced. Sponge-remains are occasionally found, 

 and include examples of Femmatites, together with two species of 

 Hyalostelia. Chert is met with locally, especially in the higher 

 portion of the Dihunopliyllum Zone, but never in any great quantity ; 

 and true beds of chert are practically absent. Eeplacement of the 

 fossils by beekite and other forms of silica is not uncommon at 

 certain horizons, the various species of Sgringopora and other 

 ■corals being especially prone to this replacement. 



Good examples occur in the Ganiarotoecliia-proava Baud, and 

 again in the Cgrtina-carhonaria Beds of the type-districts, while 

 partial* silicification of the coral-surfaces is characteristic of the 

 Seminula-gregaria Zone and the Lower Michelinia Beds in several 

 districts. In all cases, the interstratification of these silicified 

 layers with beds in which no such replacement occurs appears 

 to point conclusively to the silicification having taken place 

 ■contemporaneously with the formation, or at all events previous 

 to the solidification of the deposit in which it occurs. 



Correlation. 



The pre-Carboniferous land-surface in the Xorth-West of Eng- 

 land, upon which the rocks of Lower Carboniferous age now rest, 

 -does not appear to have been submerged until some time after the 

 Carboniferous sea had invaded the South-Western Province. This 

 is shown by the almost entire absence from the N'orth- Western 

 Province of the lower zones of the Bristol District, namely, all the 

 beds below Z^ of Dr. Vaughan's classification. The possible excep- 

 tion to this generalization : namely, the deposits found in Pinskey 

 Grill in the Ravenstonedale District, though undoubtedly repre- 

 senting the lowest horizon met with, are difficult to correlate with 

 any beds in the Bristol succession. The beds lie in a pocket in the 

 Lower Palaeozoic rocks, and are overlain by the Shap Conglomerate 

 which elsewhere forms the usual base of the Carboniferous 

 ■deposits in Westmorland. 



^ J. W. Jackson, Geol. Mag. dec. 5, vol. vii (1910) p. 78. 



