24 Notices of Memoirs — Professor T. Franldin Sihly- — 



measures that have not been detected [the true Lower and Middle 

 Coal Measures] are exceedingly thin, or are represented by the 

 upper part of the Millstone Grit, there must be an unconformity at 

 the base of the Coal Measures".* ^ 



The late Dr. A. Vauglian had proved the Lower Carboniferous age 

 of the lowest beds of the Millstone Grit near Mitcheldean.^ In 1912 

 Dr. E. A. Newell Arber, whose detailed study of the fossil plants in 

 the local Coal Measures confirmed Dr. Kidston's correhition, wrote as 

 follows: "Reviewing the present evidence lam inclined to tliink 

 that it will eventually prove that an unconformity exists a short 

 distance below the Lower Trenchard Coal perhaps a little above the 

 Sandstone vein of Iron Ore. . . . True Millstone Grits, Lower, 

 Middle, and Transition Coal Measures appear to be absent in tlie 

 Forest of Dean, so that the unconformity in question is of consider- 

 able importance." ^ 



The present author's independent investigations led him in 1912 

 to the conclusion that an unconformity at the base of the Coal 

 Measures is an important structural feature in the Forest of Dean. 

 In a short paper on the Carboniferous succession* he described the 

 Lower Carboniferous sequence near Mitcheldean, proposed the name 

 Drybrook Sandstone for the "Millstone Grit" of the district, and 

 demonstrated the reality of the intra-Carboniferous unconformity by 

 describing the persistent overstep of the Coal Measures across the 

 Drybrook Sandstone and Carboniferous Limestone, as well as by 

 other evidence. 



The author has been assisted in his later researches in the 

 district by a grant from the Government Grant Committee of the 

 Royal Society. He is permitted by the Director of the Geological 

 Survey of Great Britain to make use of information gained in the 

 course of his present investigation, as an officer of the Geological 

 Survey, of iron-ores in the Forest of Dean. 



It is well established on palaeontological evidence that the 

 Carboniferous Limestone of the Forest of Dean represents, approxi- 

 mately, the lower half only of the same formation as seen in the 

 Avon Gorge at Bristol. The zones of the Carboniferous Limestone 

 Series in the Avon Gorge, recognized by the late Dr. Vaughan, are 

 denoted, in ascending sequence, by the symbols K, Z, C, S, D. 

 The highest member of tlie Carboniferous Limestone on the north- 

 eastern borders of the Foi-est of Dean, the Whitehead Limestone, 

 represents the topmost part of C {Syrwgothyrin zone) and possibly 

 the lowest part of S (Seminula zone). The Whitehead Limestone, 

 which rarely exceeds 30 feet in thickness, gives place on the south- 

 Avestern border of the Forest to a series of dolomite-mudstones, black 

 and grey crystalline dolomites, and clays with dolomite nodules, of 

 much greater thickness ; but this series does not encroach much on 

 the Seminula zone. 



^ Geology in the Field, Jubilee Volume of the Geologists' Association, 1910, 

 p. 731. 



2 Q.J.G.S., vol. Ixi, p. 252, 1905. 



^ Phil. Trans. Eoy. Soc, vol. ccii, B, pp. 270, 277, 1912. 



■* Geol. Mag., n.s., Dec. V, Vol. IX, pp. 417-22, 1912. 



