LOWER CARBONIFEROUS ROCKS OF THE FORE3T OF DEAN. 211 



14. On the Lower Carboniferous Rocks of the Forest of Dean, 

 as represented in Typical Sections at Drybrook. By E. 

 Wethered, Esq., F.G.S., F.C.S. (Read February 7, 1883.) 



[Abridged.] 



In tbis paper I sball first refer to a series of arenaceous and 

 calcareous shales which lie between the Old Red Conglomerate and 

 Lower Limestone Shales, and which represent the true basis of the 

 Carboniferous rocks in Gloucestershire. (2) I shall refer to some 

 beds in the Lower Limestone Shales which may serve as horizons 

 for correlation with other coalfields. (3) I shall consider some 

 features in the Millstone Grit. 



1. The Old Red Conglomerate at Drybrook dips to the south-east at 

 an angle of 38°. It is made up of " vein-stone " quartz * pebbles, 

 grains of ordinary quartz, measuring about *021 of an inch in dia- 

 meter, plates of mica, and a little felspar. Of the quartz grains about 

 fourteen per cent, can be recognized as crystals ; but the rest are 

 waterworn. The grains are coated with oxide of iron ; but on this 

 being removed by acid, cavities are exposed to view, some of which 

 contain bubbles. In addition to the deposition of oxide of iron on 

 the grains, silica is often found attached to them in minute well- 

 defined and clear crystals. 



The Old Red Conglomerate passes conformably into a series of 

 loose sandy beds and light and greenish-coloured calcareous shales, 

 dark shales being absent. A section of these beds, carefully 

 measured by the late Mr. John Jones and Mr. W. C. Lucy, F.G.S., 

 showing 247 feet and 108 divisions, appeared in the Proceedings of the 

 Cotteswold Club for 1867. They are there termed " Transition Beds." 

 Sir H. De la Beche has given sections of the junction of the Lower 

 Limestone Shale with the Old Red Sandstone rocks f , and has noted 

 strata similar to the beds to which I am referring ; but they appear 

 to have been much less developed in the localities where his 

 sections were made than is the ease at Drybrook. He has, however, 

 referred them either to the Old Red Sandstone or to the Lower 

 Limestone Shales. 



The sandy beds and calcareous shales are characterized by the 

 rapid succession of beds and by the variety of colour. In order to 

 examine them minutely, five beds were selected as typical, and 

 specimens taken from them ; the first was from near the base, and 

 the others from various parts proceeding upwards. 



No. 1 was a light-coloured bed with but slight cohesion between 

 the grains composing it. It contained a considerable quantity of mica 



* This term, vein-stone quartz, is used by Sir H. De la Beche, Memoirs Geol. 

 Surv. vol. i. p. 64. 



t Memoirs of the Geol. Surv. vol. i. " Formation of Rocks in the South-west 

 of England," p. 142, and in other parts of the monograph. 



Q. J. G. S. No. 154. q 



