UPPER AND LOWER LIMESTONE SHALE. 



159 



ft. in. 



14. Gritty sandstone 1 6 



15. Red sandy marl 0 3 



16. Rubbly limestone with quartz pebbles 1 5 



17. Grit 1 6 



18. Grit 1 0 



ft. in. 



19. Grit l o 



Red and blue marl l 6 



20. Grit 0 6 



Variegated marl 3 0 



21. Grit 12 0 



22. 

 23. 

 24. 

 25. 

 26. 

 27. 

 28. 

 29. 

 30. 

 31. 

 32. 

 33. 

 34. 



Middle Limestone. 



ft. in. 



Oolitic limestone, three beds 11 0 



Red marl 0 6 



I Limestone, occasionally nodular ; partly ooli- 



f tic and contains quartz pebbles 22 0 



Encrinal limestone with Caryophyllia 8 0 



Grit, varying in thickness from 2 inches to... 1 6 



Oolitic limestone 7 0 



Oolitic limestone 1 0 



Bluish clay 0 6 



Limestone 4 6 



Limestone 2 0 



Limestone 6 0 



Middle Grit. 



ft. in. 



35. Grit 2 to 3 



36. Gritty marl 4 



37. The strata here assume a basin shape, the 



cavity being filled with alternating beds of 

 grit and marl 



38. Grit 3 



39. Limestone containing quartz pebbles 



40. Grit 4 



To the last succeed several alternations of marl and grit, but they are too much decomposed to 

 permit their line of separation to be determined. 



In Monmouthshire as at Bristol, the upper limestone shale consists of alternating beds of grit, 

 shale, and limestone, showing a complete transition from the overlying millstone grit, into the great 

 calcareous mass beneath. In a section of the strata subjacent to the coal measures of the Forest 

 of Dean, Mr. Mushett describes beds of grit with subordinate red sandstone, marl, and limestone, 

 similar to those of the coal tract near Oswestry, (see p. 145,) except that in Shropshire the red 

 sandstone is much more prevalent than in Gloucestershire. To the north of Chepstow, these and 

 other strata of thin-bedded impure limestone of dark dingy red and yellow colours, (upper limestone 

 shale, &c.,) are wrapped in contortions over the great massive limestone of the Wye, and it is to these 

 contortions, as drawn by Dr. MacCulloch, that Dr. Buckland and Mr. Conybeare have adverted. 



On the other hand, the lower limestone shale rising from beneath the same limestone, 

 appears in great force along the edges of the Old Red Sandstone of the high region 

 between Chepstow and the Usk, and has been recently laid open by cutting the new 

 road between these places. These beds very much resemble the upper limestone shale, 

 and as that rock indicates a passage into the overlying millstone grit, so do the latter 

 prove the transition into the underlying Old Red Sandstone. 



Thus in travelling from Chepstow to Earlswood Common and Wentwood, we pass from older to 

 older beds of limestone, till we reach those solid escarpments of the lower part of the formation as 

 exhibited in the ravines between Well Head and Rug's Hole 1 . These rocks are underlaid by alter- 



1 These two places are so named as being the extremities of one of those "swallows" in which streams are lost, 

 and which are so common in limestone tracts. The small brook which flows by White Mill is lost at Rug's 

 Hole and re- appears at Well Head after a subterranean course of about a mile and a half. Near Well Head 

 the carboniferous limestone is capped by a fine mass of dolomitic conglomerate. (See Map.) 



u 2 



