490 A. Sfrahan — Geological Survey of South Wales, etc. 



one tint only on the old map. At the eastern end of the field it 

 was apparent that a suitable threefold division of the strata held 

 good, the three divisions not only differing in their mineral contents, 

 but presenting such physical features as lent themselves to the 

 purposes of the geological surveyor, I wish, however, to point out 

 that no correlation is intended with the Upper, Middle, and Lower 

 Coal-measures of other fields. Not onlj'^ is it extremely improbable 

 that any representatives of the Upper Coal-measures exist, but it is 

 an open question how much of the Middle Coal-measures are present 

 in South Wales. The subdivisions referred to consist of: — 



1. An upper series of shales and felspathic sandstones with a few 

 thin seams of coal and ironstone. The sandstones are often in- 

 distinguishable from Pennant, but the series is softer on the whole 

 and forms cultivated land of flowing contour. For its base the 

 Mynyddislwyn Vein, a valuable and constant house-coal, served 

 conveniently. 



2. The Pennant Series, which in Monmouthshire is made up 

 almost wholly of hard, current-bedded, highly felspathic grit, with 

 a few thin and irapersistent coal-seams. This series forms un- 

 cultivated moorlands, intersected by deep valleys with rugged sides. 

 At its base occurs the seam variously known as the Eed Ash, 

 Tillery, Brithdir, or No. 2 Ehondda. 



3. The Lower Coal Series (Steam Coal Series of Glamorgan- 

 shire), which consists principally of shales and thin beds of quartz- 

 grit. This series contains the thickest seams of coal and the bands 

 or nodules of clay-ironstone which were formerly worked in South 

 Wales. It crops out all along the margin of the Coalfield, but is 

 exposed only in the deepest valleys or along the crests of the 

 anticlines in the more central parts. 



Through the eastern end of the field these three subdivisions are 

 readily distinguished, but they expand rapidly westwards, and at 

 the same time sandstones not to be distinguished from Pennant 

 appear in the upper part of the lower series, while measures of the 

 supra-Pennant type replace the upper grits of the Pennant group. 

 They continue, however, to form the most suitable broad divisions 

 that could have been selected, though a further subdivision may 

 become necessary in view of their increasing thickness. 



The other rock-groups have been treated on similar principles. 

 The Old Red Sandstone of Monmouthshire at once lends itself to 

 division into an upper series of grits and quartz-conglomerates, 

 a thick mass of red sandstones, and a great underlying deposit of 

 red marls with thin limestones. Special attention has been paid to 

 the relations of these subdivisions to one another in view of the 

 possibility of an unconformity having remained undetected in the 

 middle of the red strata ; but though the grits and quartz-con- 

 glomerates disappear in Brecknock, no break of any significance in 

 the sequence has yet been discovered. The conformity of the Old 

 Eed Sandstone to the Upper Silurian rocks of Usk, however, may 

 prove to be more apparent than real, and must remain an open 

 question for the present. 



