126 



JEWSTONE OR BASALTIC DYKE. 



give to the mass an appearance of being stratified. These latter divisions may indeed 

 have primarily owed their existence to the form of the stratified deposits over which 

 the basalt or lava flowed, when first emitted from its vent ; whilst the vertical or 

 columnar joints, being at right angles to the surface of the underlying deposits, must 

 have doubtless resulted from the subsequent shrinking of the molten matter, as it 

 cooled down and passed from a soft into a solid state. 



It had long been known that the basalt overlies the coal-field in thick sheets, for 

 these are penetrated by shafts, to the depth in some places of sixty-four yards ; but it 

 was not ascertained till lately that this basaltic matter had been erupted through the 

 strata of the coal measures before it overflowed them. This idea, as applied to the Clee 

 Hills, was first broached by Mr. Bakewell, who appears to have framed his hypothesis 

 upon evidence collected in a passing visit from some of the miners, respecting what 

 they termed the great "Jewstone" fault; the basalt being known in this country under 

 the name of "Jewstone." I have, therefore, great pleasure in being able to announce 

 that his conjectures were correct, and that recent workings have completely established 

 the fact, that there does exist a subterranean conduit or eruptive channel by which the 

 volcanic matter was protruded to the surface. This phenomenon has recentlybeen brought 

 into a clear light by the spirited exertions of Mr. Lewis of Knowlbury. To comprehend 

 the nature of this discovery it is necessary to refer to the map, in which is marked a 

 somewhat devious line proceeding from south-west to north-east, across the great tabular 

 mass of basalt which covers the greater part of the productive coal-field of Cornbrook. 

 This line indicates a great fault which cuts off the coal-field of Cornbrook on the south- 

 east, from that of the Hoar Edge and Horse Ditch pits on the north-west. This inter- 

 position of a wall of basalt was proved by the former workings of Mr. Botfield along 

 the north-western edge of the Cornbrook field. Another proof of basalt having been 

 erupted upon this line of fissure, was detected at a little mine of Mr. Botfield's called the 

 Hill work, where a patch of coal, situated upon and near the south-western termination 

 of this fault (containing the three principal seams,) was fairly worked out and found to 

 be based upon as well as flanked by basalt. (See PI. 30. fig. 8.) The greatest breadth 

 of this little work, viz., one hundred and fifty yards, seemed to indicate the width of a 

 subterranean channel or tube of eruption. A portion of " the Hill work" within the 

 Treen Pits liberty was also carried down to the basalt, and the beds of coal and iron- 

 stone were found to be the same as at the Treen Pits and Cornbrook, with this excep- 

 tion, that the great or upper coal was wanting. 



In this state of our knowledge respecting the probable source of the basaltic matter, 

 Mr. Lewis commenced two operations ; one to prove the width of the basaltic dyke or 

 channel of eruption ; the other to ascertain whether productive coal measures existed 

 under the portion of the basaltic sheet called the Hoar Edge. To prove the width and 

 nature of the basaltic dyke, or " Jewstone fault," between the hill work and the Treen 

 Pits, a shaft was sunk close to the side of the wall of basalt, which there rises to the 



