492 Geological Societij. 



feather off at one extremity with a l)ioken-(lovvn margin, and thus 

 complete their resemblance in piiysical features to ancient craters of 

 eruption*. The greatest superficial extent of the VVenlock formation 

 is in the neighbourhood of Walsall, where it rises both in dome- 

 shaped masses and in rectilinear ridges, running from S.S.VV. to 

 N.r^.E. parallel to the axis of the Wolverhampton coal-field, of which 

 one of these ridges forms the eastern boundary, the limestone 

 plunging beneath the coal-field at a rapid angle. The oiher ridge is 

 continuous with the new red sandstone of the Bar-beacon, and is 

 known as the Hay Head lime. In the Dudley or 10-yard coal tract 

 few works have yet proceeded downwards beneath the lower coals, 

 and hence the subjacent Silurian rocks are liltle known to the miners. 

 A remarkable and accidental discovery of a mass of limestone took 

 place recently near Dudley Port, on the rise side of a great fault, which 

 bounds the downcasts of the coal, called " Dudley Trough." Having 

 worked out the coal on the upcast side, a shaft was sunk in and upon 

 the southern side of this fault, when at a depth of 208 yards, and about 

 100 yards below the exhausted coal strata, a mass of limestone was 

 met with, which proved to be near 7 yards thick, and of very good 

 crystalline quality. Being found to extend in a form more or less ho- 

 rizontal, extensive works were promptly opened in it for the extrac- 

 tion of a rock so precious in the heart of the coal-field. When the 

 author visited it, a considerable cavity had been formed, in which no 

 trace of moisture was discernible, whilst it was known that copious 

 streams of water were flowing in the coal measures overhead. He 

 accounts for this mass of limestone being hermetically excluded from 

 the percolation of water, by the impervious nature of the Silurian shale 

 which separates the coal measures from the limestone, and by the shafts 

 being sunk in the fault itself, which, like other lines of fissure, is filled 

 up with clay and other materials, so closely compacted as to form com- 

 plete dams to water. At the north-western edge of the subterranean 

 excavation the fault was stripped, and the materials of which it is 

 composed having thinned out, the limestone was found in contact with 

 a bed of coal, the edges of which appeared bent, both the coal and 

 the limestone having a slickensides polish. By boring through the 

 limestone a second calcareous stratum was found, thus completing the 

 proofs of identity between this underground mass and that which rises 

 to the surface iii the hills of Dudley Castle and the Wren's Nest. 



In the northern or Wolverhampton field, where the whole of the 

 coal measures, even to beneath the lowest beds of ironstone, (the blue 

 Jlatg,) are traversed by shafts not exceeding 120 yards in depth, the 

 field has been proved at several points to rest on shale and impure 

 limestone, the equivalents of the Ludlow and Wenlock formations. 

 For lists of the fossils in this group of Upper Silurian rocks the author 

 refers to previous memoirs, announcing that more perfect lists will 

 shortly be laid before the public in his large work upon the Silurian 

 system. 



* See account of Valley of Woolhope for similar pliaenomena on a larger 

 scale, and with a greater number of concentric and enveloping formations. 

 — Lend, and Edinb. Phil. M;ig., vol. iv. p. ;^72. 



