220 Mnrehisori s Silurian System. 



self expresses it, from the thin, and slightly valuable coal 

 tracts around Shrewsbury, to the great productive coal field 

 of Shropshire, in which are found nearly all the members of 

 the coal formation. Here Mr. Murchison refers to the 

 labours of Mr. A. Aikin, published in the Geological Tran- 

 sactions, as well as to those of Mr. Prestwich, now in pro- 

 gress of publication in the same work. The principal and 

 most productive part of this district spreads out on the north 

 of the Severn, flanked on the south by the old red sandstone 

 and upper silurian rocks ; to the west by a thin zone 

 of lower silurian rocks, and by the trap rocks of the 

 Wrekin and Grecal hills ; but from the north-west and east 

 this tract is bounded and overlaid by the lower new red 

 sandstone ; the passage of the coal measures into that for- 

 mation is not however so clear, as in the Shrewsbury field, 

 being generally concealed by accumulations of drifted mat- 

 ter; no attempts like those of the Earl of Dartmouth (p. 18), 

 have yet been made to overcome these obstacles and follow 

 out the coal beneath the red sandstone. Here Mr. Murchi- 

 son's remarks may be regarded as an instance of how near 

 scientific induction amounts in value to the costly experi- 

 ence of practical operations in these matters. If we make 

 a transverse section from the coal works at Donnington 

 across the hills to the east, we see that where the rocks 

 are nearest to the line of red sandstone, the coal strata dip 

 to the east, or beneath the adjacent sandstone. This coin- 

 cidence of inclination in the red sandstone and coal mea- 

 sures may be observed at several points. On the north and 

 on the south, and at Brasely, Mr. Prestwich has detected 

 a passage from the upper carboniferous strata into the 

 lower new red sandstone, similar to the one described in the 

 Shrewsbury field. At Tasley, a single thin bed of im- 

 pure and poor coal is worked by windlasses, at depths 

 varying from 12 to 30 yards, and in the overlying strata 

 is a bed of limestone, about three feet thick, identical with 

 the fresh water limestone of the Shrewsbury field. This 



