26 Arber, Fossil Plants fivin the Ardwick Series. 



occur in certain other coalfields, e.g., the Potteries Coal- 

 field\ " Spirorbis Limestones " overlie beds of Middle 

 Coal Measure age in the Cumberland Coalfield, and 

 although very few, if any, plants have yet been obtained 

 from the Upper Division of the Sandstone Series of 

 Whitehaven, it is possible that this division is also the 

 equivalent of the Upper Transition Series in other coal- 

 fieldsl On the other hand, while it has been shown on 

 palseobotanical evidence that the horizon on which the 

 Spirovbis Limestones occur in certain coalfields is constant, 

 yet it has also been recently pointed out that this is not 

 always the case. In the Upper Carboniferous rocks of 

 North Staffordshire, Mr. Walcot Gibson" has shown that 

 these limestones have a wide vertical distribution. Their 

 occurrence is therefore not to be relied upon as marking 

 any particular horizon in the Carboniferous rocks, as was 

 formerly supposed, and the true position of such limestone 

 bands can only be ascertained by an examination of 

 the flora of the associated beds. The use of the term 

 " Ardwick Stage " as a horizon of wide occurrence in 

 British coalfields is misleading if founded on a particular 

 lithological type, whereas, so far as our knowledge extends, 

 it is widely applicable as a palaeobotanical subdivision of 

 the Coal Measures. 



^Kidston ('97). 



2 Arber ('03), p. 17. 



» Gibson ('Ol). 



