384 Geological Society. 



At Risca, the total thickness has diminished from 2700 feet to 

 800 feet, made up as follows : — 



C 2 + S x "j Feet. 



Cj_ > 675 Main Limestone. 



Z J 



K 125 Lower Limestone Shales. 



800 



There the Main Limestone is almost wholly dolomitic, and 

 nearly barren of fossils. Its lower portion consists of crystalline 

 dolomites, which represent contemporaneously - altered standard 

 limestones. Its upper portion, composed of dolomite - mudstones 

 with bands of crystalline dolomite and a very small thickness of 

 calcitic oolite and calcite-mudstone, is a Modiola phase extending 

 from some level in C 1 up into S r The Lower Limestone Shales 

 maintain a normal character. 



The great reduction of thickness is due to two factors, in about 

 equal degree : — 



(1) Unconformable overstep by the Millstone Grit. This cuts 

 out the Dibunophyllum beds and the Main Seminula Zone. (Overstep is 

 very gradual along most of the outcrop, but it becomes rapid at those points 

 where the zones of the Carboniferous Limestone swing more sharply north- 

 eastwards.) 



(2) Attenuation of the surviving zones. (This is rapid in the 

 area east of the Taff.) 



The change of lithological and faunal character in the zones 

 which escape overstep is due to : — 



(3) Progressive increase in the vertical extent of con- 

 temporaneous dolomitization ; supplemented, in the area east of the 

 Taff, by a great development of ikfocHoia-phase deposits in which 

 dolomite-mudstones predominate. 



The outcrop now described supplies, therefore, a key to the 

 remarkably attenuated development of the Carboniferous Lime- 

 stone Series which is known to prevail on the eastern and north- 

 eastern borders of the coalfield. Overstep and actual thinning 

 are both operating in a north-easterly direction to produce great 

 attenuation. 



The unconformity between the Carboniferous Limestone and 

 the Millstone Grit is doubtless due to the earth-movement which 

 caused flagrant unconformity between Lower and Upper Car- 

 boniferous in the Forest of Dean. 



A detailed description of the lithological and faunal succession 

 is given. The physical features of the outcrop are described, and 

 attention is drawn to the remarkably perfect adjustment of minor 

 drainage-lines to geological structure. The paper is illustrated by 

 maps on which the zonal divisions are indicated, by horizontal and 

 vertical sections, and by photographs which depict some of the 

 most interesting features of the scenery. 



