J. E. Marr — The Coniston Limestone Series. 107 



accumulation was probably very great, and its characteristic organisms 

 had time to become widely dispersed. The StaurocepJialus fauna is 

 far from being fully described, and few deposits would better repay 

 a close examination by a local geologist. The most fossiliferous 

 localities yet discovered in the north of England are the west corner 

 of Ashgill Quarry, the moorland between Skelgill and Nanny Lane, 

 Troutbeck, and, in the Cross Fell area, Swindale Beck and Billy's 

 Beck. The Echinoderms and Crustacea of the bed are particularly 

 remarkable. 



The overlying Ashgill Shales and their equivalents are fairly well 

 known in those regions where there is a passage betwixt the Ordo- 

 vician and Silurian strata. In Scotland we have similar shales 

 above the starfish-bed in Lady Burn. In North Wales blue shales ap- 

 parently referable to this horizon occur between the Bala Limestone 

 and the Hirnant Limestone. In South Wales the Eedhill beds con- 

 tain a similar fauna, and occupy the same position. In Sweden 

 the resemblance of the shales lying between the StaurocepJialus 

 Limestone and the representatives of the Stockdale Shales to our 

 Ashgill Shales is very noticeable. 



In North Wales the Hirnant Limestone is generally placed at the 

 summit of the Ordovician beds, and Mr. T. Koberts and myself have- 

 also placed the Slade Beds of South Wales in a similar position. In 

 Scandinavia, TuUberg assigns the lowest Graptolite-bearing stratum 

 above the beds containing normal Bala fossils to the Ordovician 

 system, on account of the absence of Monograptus. As it is succeeded 

 by beds containing Dimorpliograptus, it is probably the equivalent 

 of the zone of Diplograptus acuminatus of the Birkhill (Skelgill) 

 shales, and the same may be true of the Hirnant Limestone and the 

 Slade Beds. The truth is that where we have an unbroken suc- 

 cession between Ordovician and Silurian rocks, the exact line of 

 demarcation must be purely conventional. 



In the table ^ (Plate III.) showing the variations of the different 

 members of the Coniston Limestone Series, no attempt is made 

 to give an exact representation of their actual thicknesses in various- 

 localities, for in the case of beds which have been so disturbed, such 

 thicknesses, as taken by measurement on the ground, would probably 

 be incorrect. Nor is this a matter of much importance in a case 

 where volcanic outbursts are clearly seen to determine to a very^ 

 large extent the changes of thickness, when the beds are traced 

 along the outcrop. It is clear that in such cases, the position of 

 former continental masses cannot be ascertained from a study of the 

 direction of thinning out of the beds. The Ashgill Shales, however, 

 do not show any great amount of volcanic material, but appear to be 

 normal sediments. If the thickness assigned to these beds in Fairy- 

 gill, in the Sedbergh district, be an approximation to their original 

 thickness, this would show an expansion of these beds when traced 

 eastwards, and this agrees with the observations made by Professor 



^ In this table, whilst the lava-flows and more prominent ashes are inserted, no 

 attempt is made to indicate the finer volcanic material mixed with the Calcareous 

 muds of many of the beds of the Eoman Fell and Sleddale Groups. 



