PEMBROKESHIRE (GEOLOGICAL STRUCTURE OF). 



371 



a stronger proof cannot be given of the imperfect state of our knowledge of these lower 

 rocks, than the mere inspection of a map, beautifully diversified in other parts, yet in- 

 cluding under one colour, all those formations with subordinate limestones which con- 

 stitute the Silurian System, as well as a large portion of the rocks of the Cambrian 

 System. It may indeed be said, that "grauwacke" was at that time considered the 

 limit, on reaching which all stratigraphical and geological definition ceased 1 . It has 

 already been amply shown that this word should cease to be used in geological no- 

 menclature, and I shall in the following pages give further proofs that it is mineralogically 

 valueless, because rocks undistinguishable from the so-called greywacke (Silurian and 

 Cambrian) of this and other districts, occur both in the Old Red Sandstone and in 

 the Coal Measures. 



Pembrokeshire, the extreme promontory of South Wales, washed on two sides by 

 the sea, and fissured on its southern face by the deep bay of Milford Haven and its ac- 

 cessary inlets, affords extraordinary facilities for the study of its mineral structure. If 

 divided by a line running from east to west along the centre of the county, we find the 

 stratified rocks to the north of that line composed of slates, grit and shales, forming a 

 large portion of the Cambrian System ; whilst to the south the older rocks are sur- 

 mounted by the Silurian System, the Old Red Sandstone, the carboniferous limestone 

 and coal measures. The county is further diversified by a vast number of outbursts of 

 trappean rocks which have exceedingly modified the surface and altered the structure 

 of the sedimentary deposits. An inspection of the transverse section from Fishguard 

 on the north, to Pennyholt Stack on the south, explains their general relations. (PI. 35. 



f. i.) 



Coal or Culm Measures. 



The youngest sedimentary deposits of Pembroke consist of shale and sandstone, which 

 overlie the millstone grit and carboniferous or mountain limestone, and contain beds of 

 stone coal or culm. 



Before, however, we enter on the details required to explain the structure of the culm 

 measures, it may be well to state that there is no geological difference between stone 

 coal or culm, and bituminous or common coal. They are in fact mere mineral varieties of 

 the same substance, which occur in formations accumulated at the same period. The 

 coal, indeed, of the greater part of the South Welsh basin is stone coal, anthracite or 

 culm, and yet there is no field in the world where the age of the deposit is more clearly 

 marked by natural sections. 



In casting a glance over the map it will be perceived, that these carboniferous or 



1 Whatever changes may have been effected in the accompanying, in comparison with pre-existing geological 

 maps of Pembrokeshire, I must at once acknowledge the great aid I derived from being in possession of the 

 previous labours of Mr. Greenough and Mr. De la Beche, whose maps in fact are able outlines, which I have 

 endeavoured to work up to the existing state of knowledge. 



