T1IK CAl:i;i)Ml-KltOUS SYSTKM 263 



Dale and near Shap in Westmoreland, and it is these which he 

 refers to the top of the Zaphrentis zone. 



The lowest limestones are highly magnesian, and appear to 

 have been deposited under lagoon-like conditions which encouraged 

 the growth of calcareous algae, especially Solenopora, and these 

 have contributed largely to the formation of these beds. 



Most of the Vis^an limestones are rich in Foraminifera, 

 Saccammina being abundant in the D zone. The highest beds are 

 those of Humphrey Head (Morecambe Bay) and of Brampton near 

 Shap. The full thickness is probably over 3000 feet, but lessens 

 northward. 



5. Northumberland and Berwick 



From the buried ridge of older rocks which is indicated in 

 Fig. 89 the thickness of the Avonian Series again increases ; 

 thus the main mass of the Visean or Scar limestone becomes still 

 further split up by the intercalation of beds of shale and sandstone 

 till, near Alston, it has expanded from about 400 to 1600 feet, 

 with twelve distinct bands of limestone. It has, in fact, assumed a 

 facies similar to that of the overlying Yoredale Group. 



Moreover, the series increases by the incoming of lower and lower 

 beds, some of them representing Tournaisian stages, and at Alston 

 these are 1000 feet thick and about 2000 on the Scottish border. 

 This lower group contains but little limestone, and is largely 

 arenaceous, so that the name of Tuedian was given to it in 1856 

 by Mr. G. Tate, from its fine development along the valley of the 

 Tweed. 



Similarly in 1877 Professor Lebour proposed the name Bernician 

 for the compound facies which represents the higher limestones of 

 the Vise'an and the Yoredalian stage. 



Probably the most convenient provincial classification of the 

 series is that adopted in recent memoirs of the Geological Survey, 

 but this now requires to be brought into line with the zonal 

 classification of more southern areas, and with the "stage" 

 divisions which have been used in the preceding pages. The 

 zonal comparison has been accomplished in large part by Professor 

 Garwood, 9 and is indicated in the following table, except with 

 regard to the higher beds, which, being the equivalents of the 

 Yoredale Beds, must represent the highest part of the Dibuno- 

 phyllum zone if Mr. C. Johns' views are correct : 



