158 CARBONIFEROUS LIMESTONE OF MONMOUTHSHIRE, ETC. 



no portion of the girdle which surrounds the great coal basin of South Wales, but are 

 remnants of another large carboniferous tract in Pembrokeshire, to which allusion will 

 be made in a subsequent chapter. The various elevations and subsidences to which 

 the limestone has been subjected, between the Caermarthen Fans and the mouth of 

 the Towy, and the different degrees of inclination at which the strata dip, may be ob- 

 served in the accompanying maps and sections, and long details respecting them are 

 uncalled for, as they must naturally form a portion of the work of Mr. Conybeare. The 

 dismemberments in the immediate neighbourhood of the Caermarthenshire Fans, with 

 an account of two remarkable outliers, will form the subject of the next chapter. 



Additional observations on the Carboniferous Limestone of Monmouthshire, particularly on 



the Upper and Lower Limestone Shale. 



Besides the previous brief sketch of the northern, north-eastern, and western portions 

 of the carboniferous limestone of the South Welsh coal basin, there will be found details 

 respecting the development of the formation, in the subsequent chapters upon Pem- 

 brokeshire and Tortworthin Gloucestershire, in which, for the sake of clearness, all the 

 strata associated in those tracts are described in succession. But after all, there are 

 many districts occupied by this limestone as laid down in the map, to which little 

 further allusion will be made. Before, however, we enter upon the consideration of the 

 Old Red Sandstone, I must direct attention to those places in Monmouthshire in which 

 there are the best examples of the beds of passage between the limestone and the mill- 

 stone grit, and through the lower limestone shale into the Old Red Sandstone ; as it is 

 particularly with the aid of such examples that general portions of the tabular view 

 have been filled up. The tracts of carboniferous limestone specially adverted to, are 

 those in Gloucestershire and Monmouthshire, which lie between the great South Welsh 

 coal basin on the one side, and the coal-fields of Bristol and Forest of Dean on the 

 other, and occupy the splendid gorges of the Wye near Chepstow. For much valuable 

 information respecting these limestones, I refer the reader to the valuable memoir of 

 Dr. Buckland and Mr. Conybeare on the Bristol coal-field, with which these and other 

 carboniferous rocks of the adjacent districts are placed in parallel. 



Of the Upper Limestone shale, however, as seen in the gorge of the Avon at Clifton, I here 

 subjoin an instructive and accurate detailed section by Mr. Lonsdale, made to illustrate a suite of 

 specimens in the Bath Institution, which though never previously printed, appears to me to be of 

 too great value not to be made public. 



Lower Grit. 



ft. in. 



1. Rubbly marl 16 0 



2. Highly ferruginous sandstone 2 0 



3. Limestone more or less oolitic 10 0 



4. Rubbly limestone mixed with marl 8 0 



5. Variegated marl 8 0 



6. Oolitic limestone ; in the superior part of the 



bed are small quartz pebbles 1 6 



ft. in. 



7. Grit more or less fissile 5 6 



8. A small-grained siliceous conglomerate 0 4 



9. Oolitic limestone 2 6 



10. Rubbly oolite containing quartz pebbles 4 0 



11. Red marl 1 6 



12. Hard red sandstone 



13. Finely-grained sandstone 



