Dr. T. F. Sibil/— The Forest of Bean Coalfield. 417 



dry up during several months of the year. In the beds of these 

 streams concretions of limonite appear to be forming at the present 

 time. Similar concretions are found in the alluvial sands, and in 

 many places a ferruginous cement has hardened the upper layers of 

 the latter. These phenomena are probably due to the evaporation of 

 iron-bearing solutions brought near the surface, with consequent 

 deposition of the hydrated oxides. The origin of laterites is a much- 

 discussed problem, but we believe that many of the Mozambique 

 varieties have originated in this way, a view in support of which we 

 hope to present fuller evidence on a future occasion. 



YI. — The Caeboniferous SuccessiojST in the Forest of Dean- 

 Coalfield. 



By T. Franklin Sibly, D.Sc, F.G.S., Lecturer on Geology in 

 King's College, London. 



A. Introduction. 



B. The Lower Carboniferous Eocks. 



C. The Intra-Carboniferous Unconformity. 



A. iNTRODtrCTION. 



rpHE following preliminary account is presented as the result of 

 JL several weeks of field-work on the northern and eastern margins 

 of the Forest of Dean Coalfield, carried out partly in 1910 and partly 

 in the present year. The area which. I have examined lies almost 

 wholly within Sheet 43 S.E. of the I inch Geological Survey Map 

 (Old Series), but extends a sliort distance into Sheet 43 S.W. In 

 portions of this area, the Lower Carboniferous rocks have been studied 

 in detail and mapped. The Coal Measures, however, have as yet been 

 examined only in their relation to the Lower Carboniferous strata. 

 The present communication is, therefore, confined to a demonstration 

 of the important unconformity which exists between the Coal 

 Measures and the Lower Carboniferous rocks, together with a brief 

 description of the latter. I am continuing my investigations, and 

 I hope to give at some future date a more comprehensive account of 

 the geology of the coalfield. In the meantime, the following outline 

 may be of interest. 



The Carboniferous succession in the north and east of the coalfield 

 comprises the following members : — 



Lower Carboniferous. 



1. Carboniferous Limestone Series. The strata of this division 

 succeed the Old Red Sandstone with perfect conformity, and are 

 themselves overlain conformably by 



2. The Bryhrook Sandstone. A thick sandstone formation, the 

 ' Millstone Grit ' of previous writers. 



TJppee Carboniferous. 



3. Coal Measures. As will be shown, the strata of this series lie 

 unconformably, and in some places with great discordance of dip, 

 upon the Lower Carboniferous rocks (2) and (1). 



DECADE V. — VOL. IX. — NO. IX. 27 



