258 



Martinis Geological Memoir^ and 



sketch of the Nailsea coal-field, near Bristol, on the same 

 authority, {fig, 119.) This concentric disposition, so common to 

 the carboniferous rocks, seems to indicate a degree of flexibility 

 in the materials, yielding to vast pressure, and mechanical force. 

 The hypothesis of subsidence alone, appears inadequate to pro- 

 duce the phenomena of curvature, so frequently witnessed in 

 coal basins. 



Nailsea. 



120 



In strata of a much more recent date, the occasional basin- 

 shaped arrangement is per- 

 ceived. We might illustrate 

 this by the well-known sec- 

 tion of the basin of London^ 

 {Jig, 120.), as it is termed,'' 

 and by that of Hampshire, 

 both of which are occupied with deposits of the most recent 

 origin in our geological system. 



It has been observed that some of the ancient strata exhi- 

 bit marks of greater disturbance and distortion than those of 

 later formation. Of the former class, numerous singular illus- 

 trations may be seen in the transition slate rocks. The rock 

 distinguished by the name of mountain limestone, is particu- 

 larly remarkable for its contortions ; an example may be men- 

 tioned, at the upper part of the Vale of Neath, in the Dynais 

 rock, where its once flexible beds are twisted into the form of 

 natural arches. 



We transfer, v,^^. i^i 



from Mr. De la 

 Beche*s paper 

 on the Geology 

 of South Pem- 

 hrokeshire^ a 

 sketch (/^. 121. 

 of an appearance 

 of this kind, in 

 the coal mea- si 

 sures, at Monk's 

 Stone Point, near Tenby. 



