256 Messrs. Bucrland's and Conybeare's Observations on the 



but to the east of this meridian as far as Mells they dip to the south, appearing 

 to sink beneath the great substratum of limestone ; so that they have been turned 

 over and throw^n backwards, and often to such a degree, as to lie more than 45° 

 to the north of the vertical line. This inversion of dip extends through a tract 

 4 miles long, and is proved in more than ten distinct collieries ; and the con- 

 sequence of it isj that those seams which lie the highest in the shafts to the 

 west of Pitcot are found the lowest in the shafts to the east of that place. 



These changes of dip are frequently attended with contortions as abrupt and 

 striking as those in the primitive slate rocks, especially in the southern series 

 of beds, which are often broken and twisted most capriciously. At Bilborough 

 colliery in Vobster the flexures are so great as to twist the strata like the letter 

 Z, so that the same coal-seam is thrice cut through by the same perpendicular 

 shaft ; and it is a common occurrence in this line of collieries for this to happen 

 twice. In the adjacent limestone of the Mendips the strata though highly 

 inclined, and often vertical, are generally plane and parallel, and the millstone 

 grit usually conforms in this respect to the limestone ; the contortions being- 

 confined to the softer argillaceous strata, as is the case in the isle of Purbeck 

 on the coast of Dorsetshire [see Mr. Webster's representation and account of 

 these contorted strata in Sir H. Englefield's work on the Isle of Wight]. We 

 have already noticed, in our account of the Mendip chain, the occurrence of 

 two parallel faults near Vobster, which cause the limestone with its superjacent 

 coal-measures to trap up twice in succession. The lesser faults of this district 

 cannot so easily be traced, in consequence of their number and complication^ 

 and of the convolutions to which the whole mass of strata is subject. 



In the Clapton coal-field, we cannot pronounce decisively to which division 

 of the coal-measures the seams of the subsided mass belong ; the workings 

 having been so long disused that we have not been able to procure any ac- 

 count of them. We are inclined however to refer them to the lower coal-shale. 



3. The Pennant grit consists of thick stratified masses of very fissile sand- 

 stone, having mica, decayed felspar, and carbonaceous vegetable fragments 

 abundantly disseminated through it, and varying in colour from greenish gray 

 to dark brick-red. The thick beds of grit alternate with thin strata of shale 

 and coal. It is known by the name of Pennant stone^ and is quarried largely 

 for paving and building throughout the district*. The total thickness of this 

 series cannot be less than four or five hundred feet ; and is very possibly 

 greater. 



The Pennant forms a zone around the upper coal-shale within the troughs 

 on either side of Kingswood. In the northern trough it circles around Coalpit- 



* All the hard and fissile coal-grits are indiscriminately called Pennant by the miners. 



