TICK CARBONIFEROUS SYSTKM 257 



The true position of the Holywell shales was first indicated by 

 Dr. Hinde, and the group is undoubtedly the equivalent of hifl 

 Fendleside Series. In Flintshire it consists, in the lower part, of 

 black shale.s witli layers of black limestone and chert, yielding the 

 bivalves Posidonomya and Pterinopecten, while the higher part is 

 composed of soft shales in which Posidoniella Icevis is common. 



Southwards all these beds decrease in thickness rather rapidly, 

 and they finally thin out south of Oswestry, only the higher part 

 of the limestone with Prod, giyanteus appearing again for a short 

 distance in Shropshire, near Lilleshall. Westward the whole series 

 is faulted down into the Vale of Choyd, and portions of it occur 

 along the coast ; thus, in the Orme's Head promontory near 

 Llandudno, the greater part of the limestone series is found with 

 a thickness of 1150 feet, though neither the base nor the summit 

 is seen. In Anglesey tracts occur on the border of the Menai 

 Strait, and a larger one is faulted into the central part of the 

 island ; the lowest limestones here belong to the white division, 

 and in places they include beds of white sandstone and conglomerate 

 from 90 to 100 feet thick. 



In North Staffordshire and Derbyshire the zonal succession 

 has recently been determined by Mr. Sibly, and found to be as 

 follows : 



Feet. 



D 3 4. Shales and thin beds of black limestone (Yoredale Beds) . 400 

 D._. 3. Grey and white limestones containing layers of chert (sub- 

 zone of Lonsdaleia jtoriformis) ..... 600 



(2. Massive white limestones with interbedded lava-flows (toad- 

 DJ-! stones) = Dibunophylluin Beds . ..... 600 



( 1. Dark limestones seen for 800 



2400 



The actual base is nowhere exposed, and consequently the total 

 thickness of the series is unknown, but it is doubtful whether it 

 exceeds 3000 feet even in the centre of the area. The whole of 

 the series thins out to the south-west before reaching the South 

 Staffordshire coalfield, but continues toward the south-east beneath 

 the small Leicestershire coalfield and wraps round the pre-Cambrian 

 rocks of Charnwood Forest 



The limestones which compose the " Mountain limestone " of 

 Derbyshire vary much in lithological character, 4 some consisting 

 mainly of broken fragments of crinoids and shells, others of corals 

 and coral debris, and others being compact and foraminiferal, while 

 in many parts the organisms have been obscured or destroyed by 

 the chemical changes of marmorisation or dolomitisation. 



The upper surface of D._, shows signs of contemporaneous 



S 



