488 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



around you, you are really between two parallel vertical dykes of a 

 mucli later age, which are probably continued through many miles 

 of the Mendip district. 



Eeturning to Holwell, and following the valley in its northern 

 direction to Nunney, the Carboniferous Limestone is worked near 

 the Castle, and again a large dyke of Liassic age traverses it from 

 east to west and passes under the Inferior Oolite immediately beyond. 

 Towards Eggford the Carboniferous limestones are rather more con- 

 tinuous than is shown in the Ordnance Map, as they may be traced on 

 the eastern side of the Whatley road under the Oolite. An outcrop of 

 Carboniferous Limestone in a pasture-field under the roadway shows 

 that it is entirely an oolitic limestone of a much coarser kind than is 

 usually seen; and the rock is much mineralized, occasionally contain- 

 ing galena. Resting on the limestone are traces of the Liassic deposit 

 noticed at Whatley. In a field above, a small shaft was sunk down 

 to stratified beds of Inferior Oolite, when, resting on the latter beds, 

 a rubbly deposit of sulphate of barytes, calamine, with a mixture of 

 galena was passed through, of about a foot in thickness, these minerals 

 being unquestionably of Oolitic age. 



4. Sections in the Vallis. — Crossing the turnpike at Eggford, a 

 roadway at once leads into the Yallis ; and a short way on may be 

 observed a fine exposure of Carboniferous Limestone, the Fullers' 

 Earth in this direction encroaching upon it. On the southern side the 

 stone first seen is the end of a vein with a tendency to underlie to the 

 south ; it rested against the outcrop of the Carboniferous Limestone 

 before the quarry was worked back from this point. It is composed 

 of diiferent layers of variegated honeycombed limestone. Following 

 this band towards the face of the quarry, there rests upon it thickly 

 bedded masses of Inferior Oolite, containing abundance of Limapec- 

 tiniformis, Lima gibbosa, Echini, &c. In the southern corner and 

 leading up to the top of the quarry, the Inferior Oolite is laid dovm 

 upon the ancient Carboniferous-limestone sea-bottom, and has so 

 accommodated itself to any inequalities in its surface as to make it ex- 

 ceedingly difficult to determine where the one formation begins or the 

 other ends. So intimately united are these^ unconformable deposits 

 that the same hand specimen may show portions of each, with Litho- 

 domi, of Oolitic or any intervening age, still retained in their burrows 

 in the surface of the Carboniferous Limestone *. In this valley, as 

 in all the others described, there can be little doubt that these lime- 

 stones formed the sea-bottom during a greatly extended period. In 

 the face of this section are several irregular vertical fissures leading 

 down from the top, the first one being filled with a ferruginous or ochre- 

 ous sand. This dyke thickens towards the bottom of the quarry, but, 

 before reaching it, is apparently cut off by the limestone ; it then for 

 a short distance takes the plane of the bedding and again passes down 

 vertically ; at this point it is divided in the centre by a siliceous band 

 1 foot thick, each side of which is composed of a ferruginous deposit 



* This had been previously noticed by Sir Henry de la Beche, in his yaluable 

 Memoir on the ' ' Formation of Eocks in South Wales and South-western Eng- 

 land," p. 290. 



