YoL 6t > .'] SILURIAN INLIER IN THE EASTERN MENDIPS. 217 



13. A Silurian Inlier in the Eastern Mendips. By Prof. Sidney 

 Hugh Reynolds, M.A., P.G.S. (Read March 13th, 1907.) 



[Plate XVIII —Microscope-Sections.] 



Contents. 



Page 



I. Introduction 217 



II. Description of the Exposures 219 



(1) The Lava or Trap. 



(2) The normal Tuffs and the associated Fossiliferous Eocks. 



(3) The Coarse Ashy Conglomerate. 



III. The Petrographical Characters of the Eocks 230 



(1) The Lava or Trap. 



(2) The normal Tuffs. 



(3) The Coarse Ashy Conglomerate. 



TV. The Mutual Eelations and Nature of the Eocks 234 



V. Summary and Conclusions 238 



I. Introduction. 



The Mendip Hills consist of four periclinal upfolds of Carboniferous 

 Limestone, arranged en echelon from north-west to south-east. 

 Each pericline includes a core of Old Red Sandstone, and the 

 south-eastern or Beacon-Hill pericline further shows a series of 

 igneous rocks. It is with these that the present paper deals. The 

 existence of igneous rocks in the Eastern Mendips was first noted 

 by Charles Moore, 1 who described them as 



' a basaltic dyke of considerable thickness emerging from beneath the Old Eed 

 Sandstone at East End near Stoke Lane.' 



He considered that, from the general physical character of the 

 Mendips, it was not improbable that the dyke might be co-exten- 

 sive with their range. He not only attributed the upheaval of the 

 whole Mendip range to the intrusion of this igneous mass, but also 

 considered that it was responsible for the remarkable inverted 

 character of the Carboniferous beds at Luckington, where the 

 Coal-Measures are worked under the Carboniferous Limestone. 

 John Morris 2 refers to the rock at Stoke Lane, as ' 



' a dyke of considerable thickness, emerging from beneath the Old Eed Sand- 

 stone, occurring as bosses in the field, but, traced for some distance over 

 the district, it is conglomeratic in places, and pronounced by Mr. D. Forbes 

 to be dolerite.' 



The igneous rocks are not shown in Sanders's map of the 

 Bristol Coalfield (published in 1864), but appear in the map of 

 the Geological Survey (1884) as a series of isolated patches ex- 

 tending from Downhead on the east, to Beacon Plantation, south- 

 west of Stoke Lane, on the west, a distance of about 3 miles. 



1 Quart. Journ. Geol. Soc. vol. xxiii (1867) pp. 451-52. 



2 Geol. Mag. vol. v (1868) p. 236. 



Q. J. G. S. No. 250. q 



