380
MILLSTONE GRIT AND CARBONIFEROUS LIMESTONE.
geological series as the Pembroke field, i.e. above the millstone grit and carboniferouslimestone. (See pp. 85, 103 and 116. 1 )
Millstone Grit .—Little need be said of this formation, for it is identical with beds of the sameage in Shropshire and other parts of South Wales, and contains no organic remains nor coal. Ineastern Pembroke, particularly in the Marros Mountain and adjoining lands, it is fully developedand moderately inclined, passing upwards into the sandy, flaglike beds which form the bottom ofthe 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 theCleddau, where the underlying limestone is very thin, the lowest beds of themillstone grit consist of a very hard, thick-bedded chert, which caps the limestone at HaroldstoneIssels. The same rock appears on both banks at Langam Ferry, and is again found near Johnstonand on the edge of the Poorfield Common near Haverfordwest. It is the cleggir of the Pembrokequarrier, a most intractable building stone but excellent for the roads. Where the limestone tapersout, the cleggir is a good index of its position. This rock plunges under the great mass ofmillstone grit which occupies the Poorfield Common to the west of Haverford. It is the upperportion of that formation which, advancing to the coast at Haroldstone Nose, throws off on eachside (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 northernedge of the coal-field, it dips to the south, at angles not exceeding 30° or 35°, and in the westernportion of the northern zone (in the quarries of Poorfield, Haroldstone Issels, Hampton andMilling) it either abuts against or rests uneonformably 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 atHampton near Usmaeston, on the left bank of the Haverford river, it rises in a dome with a veryslight inclination, whilst on the opposite bank at Haroldstone it is thrown off at 30° to 35° south, andconsequently 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 thewest of Maudlin Bridge (where it is seen for the last time beneath the millstone grit), but it merelyprotrudes 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 inthe 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 metwith 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 iu previous geological maps as consisting of greywacke, a mistake doubtlesscaused by the lithological aspect of this millstone grit. (See Geol . Trans, vol. ii. pp. 10 & 20.) (PI. 2. f. 1.)