418 D)\ T. F. Sihly—The Fored of Dean Coalfield. 



B. The Lower Cakbonifeeous Eocks. 

 Carhoniferous Limestone ISeries. 



Mr. E. AVethered has given some description of the strata of this 

 series in three papers.^ More recently Dr. A. Yaughan has given ^ 

 an outline of the sequence in the Mitcheldean district, on the north- 

 eastern margin of the coalfield, and a correlation of this sequence 

 with the Avonian of the Bristol area. My own studies confi.rm 

 Dr. Vaughan's conclusion that the top of the Carboniferous Limestone 

 Series in the Forest of Dean lies far below the summit of the Avonian ; 

 but the features of tlie series as I have observed them differ in several 

 important particulars from those outlined by him. 



Tlie Carboniferous Limestone Series is readily capable of a three- 

 fold subdivision on lithological grounds. The following table gives 

 the approximate thickness of each subdivision in tlie Mitcheldean 

 district, and a correlation with the zones of the Avonian. 



Avonian Zones, 

 (c) ^V^l^te^^ead Limestone, 20 to 50 feet, 

 (b) Main Limestone, about 320 feet 



(a) Loiver Limestone Shales, about 180 feet 



Lower Limestone Shales. This subdivision represents the Cleistopora- 

 zone (K), and probably includes the base of the Zaphrentis-zone 

 (Z) also. It comprises limestones and shales of very varied character, 

 and passes down conformably into the sandstones which form the 

 top of the Old Red Sandstone. Its lowest member is a thick band of 

 limestone, often oolitic and gritty. At the junction with the Old 

 Bed Sandstone an alternation of limestone with sandstone occurs 

 throughout a small thickness of beds, but there is no appreciable 

 development of a transition-series comparable, either faunally or 

 lithologically, with the Modiola-'phnse at the base of the Avonian in 

 the Avon section. Tlie subzones Kj and Kj can be distinguished, 

 but in each sub-zone the faunal assemblage presents considerable 

 differences to the corresponding assemblage in the Bristol and Mendip 

 areas. 



An important feature of the Cletstopora-zone in this area is the 

 strong development of Modiola - phase (calcareous lagoon - phase) 

 deposits in the middle and upper portions of the zone. These 

 deposits are of exceedingly varied lithological character. As regards 



' " On the Lower Carboniferous Rocks of the Forest of Dean, as represented 

 in Typical Sections at Drybrook " : Q..J.G.S., vol. xxxix, p. 211, 1883. " On 

 the Structure and Organisms of the Lower Limestone Shales, Carboniferous 

 Limestone, and Upper Limestones of the Forest of Dean " : Geol. Mag., 

 Dec. Ill, Vol. Ill, p. 530, 1886. " On the Lower Divisions of the Carboniferous 

 Eocks in the Forest of Dean " : Proc. Geol. Assoc., vol. x, p. 510, 1887-8. 



• Q.J.G.S., vol. Ixi, pp. 251-2, 1905. 



