380 



MILLSTONE GRIT AND CARBONIFEROUS LIMESTONE. 



geological series as the Pembroke field, i.e. above the millstone grit and carboniferous 

 limestone. (See pp. 85, 103 and 116. 1 ) 



Millstone Grit. — Little need be said of this formation, for it is identical with beds of the same 

 age in Shropshire and other parts of South Wales, and contains no organic remains nor coal. In 

 eastern Pembroke, particularly in the Marros Mountain and adjoining lands, it is fully developed 

 and moderately inclined, passing upwards into the sandy, flaglike beds which form the bottom of 

 the culm-field, and downwards into the carboniferous limestones which represent the upper lime- 

 stone shale. It is seldom a conglomerate, being for the most part a hard, siliceous, whitish sand- 

 stone, graduating into grits both fine and coarse. 



On the banks of the Cleddau, where the underlying limestone is very thin, the lowest beds of the 

 millstone grit consist of a very hard, thick-bedded chert, which caps the limestone at Haroldstone 

 Issels. The same rock appears on both banks at Langam Ferry, and is again found near Johnston 

 and on the edge of the Poorfield Common near Haverfordwest. It is the cleggir of the Pembroke 

 quarrier, a most intractable building stone but excellent for the roads. Where the limestone tapers 

 out, the cleggir is a good index of its position. This rock plunges under the great mass of 

 millstone grit which occupies the Poorfield Common to the west of Haverford. It is the upper 

 portion of that formation which, advancing to the coast at Haroldstone Nose, throws off on each 

 side (as before described) the lower and slightly productive beds of the culm measures 2 . 



Carboniferous Limestone. 



This formation dips beneath the millstone grit and forms a girdle round the great eastern district, 

 but thins out to small patches on the west. Whenever the limestone appears along the northern 

 edge of the coal-field, it dips to the south, at angles not exceeding 30° or 35°, and in the western 

 portion of the northern zone (in the quarries of Poorfield, Haroldstone Issels, Hampton and 

 Milling) it either abuts against or rests unconformably upon strata of the Silurian System. To in- 

 dicate the disjointed condition of the limestone in this part of its course, we may notice that at 

 Hampton near Usmaeston, on the left bank of the Haverford river, it rises in a dome with a very 

 slight inclination, whilst on the opposite bank at Haroldstone it is thrown off at 30° to 35° south, and 

 consequently the one mass is not in a continuous alignment with the other, but is projected consi- 

 derably to the south. Still less is the rock traceable as a persistent zone from Haroldstone to the 

 west of Maudlin Bridge (where it is seen for the last time beneath the millstone grit), but it merely 

 protrudes on that one point, the intermediate tract being occupied by the overlying grits of the coal 



1 Several of the plants common to the Salopian and Pembroke coal fields are the most abundant species in 

 the culm measures of Devon. On this point Professor Lindley, after a re- examination of many specimens, 

 thus expresses himself : " Respecting the Devon culm plants, I have still the same observation to make as be- 

 fore. I have looked them over carefully, and I do not see one single species which might not have been met 

 with at Newcastle, with the exception of two round compressed bean-like bodies, which, if of vegetable origin, 

 are unknown to me." 



2 This is the tract marked in previous geological maps as consisting of greywacke, a mistake doubtless 

 caused by the lithological aspect of this millstone grit. (See Geol. Trans, vol. ii. pp. 10 & 20.) (PI. 2. f. 1.) 



