374 



CULM MEASURES OF ST. BRIDE'S BAY 



Now if any geologist was landed in St. Bride's Bay, and not having traced these 

 coal measures from situations where they lie in regular order of superposition, was sud- 

 denly required to determine the relations of the various strata presented to him (as 

 represented PI. 35. f. 7.), he might well draw the most erroneous conclusions concern- 

 ing the age of the culm beds, and pronounce them to be a part of what has hitherto 

 been called the greywacke series 1 . 



Not only have these strata been dislocated and contorted in the highest degree, (much 

 exceeding the irregularities east of Tenby ;) but formations elsewhere separated by dif- 

 ferent deposits, are brought into juxtaposition. In the first place the carboniferous or 

 mountain limestone is totally wanting ; and the sandy strata, representing the millstone 

 grit, often thin out, as expressed in wood-cut 73, p. 375. In this case the culm 

 bearing beds repose at once and almost in conformable position on black schist of 

 the Lower Silurian Rocks, much resembling in mineral characters, the culm shale ; 

 while near Brawdy the culm field rests upon, and has the appearance of graduating into 

 the Cambrian System. (Wood-cut 74, p. 375.) 



Now in both the last situations, there is so little appearance of wnconformability 

 between the culm strata and the Lower Silurian and Cambrian Rocks, and in many 

 respects so much the appearance of a passage downwards, that any one, unfurnished 

 with the key to solve such anomalies, might consider the culm strata to be part of 

 the Cambrian and Silurian Systems. (See wood-cuts on the next page.) Yet nothing 

 would be more erroneous than this conclusion ; such culm strata being in truth the 

 coal measures above the millstone grit, represented throughout the map by the letter g. 

 In the county of Pembroke, for example, we can walk upon the same beds, from the 

 district where they repose regularly upon the millstone grit (as represented in the 

 upper wood-cut,) till we find them in this western tract in the anomalous positions 

 expressed in the lower figures. (See Map.) 



Such apparent anomalies are not, however, difficult of explanation, for geologists need 

 scarcely be told, that when the formations which usually support coal-fields are absent, 

 the lower carbonaceous strata, must of necessity resemble the rocks they rest upon, and 

 out of whose detritus they have been formed. This reasoning, indeed, applies to rocks 

 of all epochs, for numberless are the cases where strata, even of the tertiary age, 

 resting directly upon gneiss and other primary rocks, assume the lithological characters 



1 These remarkable phenomena in Pembrokeshire first led me to suppose, that the views previously entertained 

 of the structure of Devon were erroneous, and the above sections were those which I communicated to 

 Professor Sedgwick, before we examined that county, with the view of determining whether the Devon culm 

 strata were, as was then contended, subordinate to the so called " grauwacke"; or whether they were, like those 

 of western Pembroke, of the age of the true coal measures, lying in an irregular trough upon various ancient 

 rocks, into which they sometimes appeared (though deceptively) to pass downwards. The results at which we 

 have arrived are now before the public. We had no hesitation in identifying the culm strata of Devon with 

 those of Pembrokeshire and other coal-fields, See Proceedings of the Geological Society, vol. ii. p. 556. 



