Page:VCH Herefordshire 1.djvu/72

 A HISTORY OF HEREFORDSHIRE the marginal portion, however, which is much coarser grained, oHvine was present. The baked nature of the Lower Old Red marls — so well described by Murchison ^^ — shows that the intrusion occurred in post-Old Red times; but whether it was late Carboniferous or Tertiary cannot be ascertained. Carboniferous System Rocks belonging to this important system occupy but a small area in this county, occurring in the extreme south-east. They are part of the north- western fringe of the Forest of Dean Coalfield, and form the capping to the hills visible from Ross in a southerly and south-easterly direction. Where the system is complete in the west of England, as for example in the neighbourhood of Bristol,'^ it admits of the following subdivision on lithological grounds : Lower Limestone Shales, Mountain or Carboniferous Limestone, Upper Limestone Shales, Millstone Grit, Lower Coal Measures, Pennant Grit, and Upper Coal Measures. In a general way the same sub- divisions can be recognized in the Forest of Dean district, but no Carboni- ferous Beds of later date than the bottom beds of the Lower Coal Measures are found in this county. Previous to 1905 the attempts at correlating the Carboniferous rocks of various districts was largely based upon lithic structure ; but as might be expected little real progress was made. Recently, Dr. A. Vaughan en- deavoured to correlate the beds by means of their brachiopods and corals.'^ Instead of attempting too minute a correlation, he selected genera for the zonal- indexes, and ' gentes,' or ' aggregates of all the species which possess, in common, a large number of essential properties, and are continuously related in space or time.' He made five main zones, those characterized by (i) Dibunophyllum, (2) Seminula, (3) Syringothyris, (4) Zaphrentis, and (5) Cleisto- pora. He has given the term ' Avonian ' to the Carboniferous Limestone Series of the South of England, and groups the first-two named zones as the ' KidweUian,' and the other three as ' Clevedonian.' Dr. Vaughan himself visited many comparatively widely-separated areas to test the value of his proposals, and since then many other geologists have been able to interpret the true succession of the Carboniferous rocks of their districts, and to correlate the beds with their equivalents exposed in the cHfFs of the gorge of the Avon at Clifton. Dr. Vaughan visited some of the sections in the Herefordshire portion of the Forest of Dean district in order to compare the faunal succession with that at Bristol. He found that it was substantially the same, but the lithic characters of the rocks in the two districts were noticeably different. In the first place, the formation of Hmestone continued longer in the Bristol than in the Forest district, because in the former the Millstone Grit comes above the Dibunophyllum Zone, but in the latter— certainly the lowest beds of the ' Grit '— are of mt-Dibunophyllum Zone date, belonging to the Upper Seminula Zone. Interstratified in dolomites, and also belonging to the Upper Seminula Zone, is ^ Silurian Sptm (1839), pp. 185-6. '' Handbook Brit. Assoc. Bristol (1898), pp. 14-19. » Quart. Journ. Geol. Soc. Ixi (1905), pp. 181-307 and pis. xxu-xxix. 24