DISTRIBUTION. 213 



say that any Lamellibranch has any special or definite importance as a zonal 

 index in the Carboniferous Limestone Series. 



The oft-repeated localities of Thorpe Cloud, Park Hill, and Castleton, Derby- 

 shire; Narrowdale and Wetton, Staffordshire; Settle; Hill Bolton ; Hill Stebden, 

 Yorkshire ; Clitheroe, Lancashire ; and Poolvash, Isle of Man, I consider to be at 

 or about the same horizon. The fossil beds in all these localities occur at the top 

 of the series, almost immediately below the base of the Pendleside Series. It is 

 surprising how rare are macroscopic fossils of any sort in the rest of the thick 

 Limestone of the North Midlands. Of course their absence may be due to meta- 

 somatic changes, such as we know go on below the surface in coral reefs. Where 

 the Limestone is split up into several beds by intercalations of Shale and Sand- 

 stone, and the Yoredale phase is well developed, fossils are found more frequently 

 in all the Limestones, but never in very great profusion. Fossils are much more 

 common in the shales between the Limestones, but here the mud-loving genera of 

 Nuculidae, and some pectens, replace the other genera which were suited 

 by the mudless waters of the sea in which the Carboniferous Limestone was laid 

 down. 



The shales and sandstones of the Yoredale Series indicate land erosion, and, 

 consequently, the area in which they are found corresponds to the more or less 

 semicircular or pyriform area opposite the mouth of a river which receives 

 the detrital material brought down by its waters. Hence by careful mapping 

 of areas of detrital and organic deposits, with measurements of the varying 

 thicknesses of the beds, a fairly accurate idea can be obtained of the physical 

 geography of the period of any deposit, and of the habitus of the faunas in them. 



It is found that detrital beds increase in number and thickness to the north 

 and north-east ; that during the deposit of the Limestone of the Midlands no detrital 

 deposits of any amount were laid down south of a line from Grassington in 

 Craven to the south part of the Isle of Man ; that after the completion of the 

 great Limestone deposit in the Midlands detrital measures, first shales and impure 

 organic black Limestones, representing the lighter forms of detritus which would 

 have been carried farther out to sea, and finally grits of various degrees of 

 coarseness, were laid down over the Limestone area. Hence it may be inferred 

 that the Carboniferous continent existed to the north and east, with probably 

 islands in the Lake Country and a long Midland east to west ridge of land 

 separating the basin of the South Wales, Mendip, and Culm areas from that of 

 the Midlands. The proximity to land, and the gradual filling up and shallowing 

 of the sea bed from east to west, accounts for the varying character of the 

 Carboniferous deposits in different parts of Great Britain, and for the peculiar 

 distribution and repetition of faunas, which followed isobathymetric lines to a huge 

 extent. 



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