GEOLOGY OF PAET OF SOUTH DUKHAM. 167 



coal-field to have taken place during various periods — probably 

 from the latter part of the Carboniferous to some part of the 

 Mesozoic time, or possibly later. 



COAL MEASURES BELOW THE BROCKWELL SEAM. 



The Measures between the "Brockwell" and the first of the 

 grit rocks contain only thin seams of coal, which as yet are 

 hardly in themselves to be wrought at a profit. Two of the 

 coals, named the "Victoria" and "Marshall Green" seams, 

 have been tried, and to some extent worked near Witton-le- 

 Wear, in the Beechburn Valley, and on Cockfield Fell, and they 

 are said to be of good quality. Other thin coals occur, some of 

 which are associated with good fireclays ; and occasionally beds 

 of hard, fine grained siliceous sandstone form the floor of the 

 coals, which are termed "ganister," from their resemblance, 

 we suppose, to a similar rock forming the floors of the thin coals 

 at the base of the Lancashire and Yorkshire coal-fields. This 

 sandstone forms an admirable firestone, and it is used, mixed 

 with fireclay, in the manufacture of a superior class of fire-bricks, 

 locally known as "ganister" bricks. 



Some interesting sections of these Measures are to be seen in 

 the valley of the Gaunless west of Cockfield, where certain of 

 the beds are worked for flags, roofing slates, and fireclay. The 

 outcrop of some of the thin coals are also to be observed there. 

 One of the latter, supposed to be the Victoria seam, is exposed 

 on the S. bank of the Gaunless, above Haggerleases Lane, where 

 it affords the following section : — 



Feet. In. 

 White and yellow sandstone 6 



COAL, with half -inch band near centre 1 6 



Coarse black shale l£ 



Greyish white siliceous sandstone, with many roots and root* 



lets of Stigmaria „.... 1 6 



The Oaky Bank Quarry, on the N. bank of the stream, near 

 Gibbsineese, more especially offers a good section of what are 

 probably the middle beds of the series, including one of the thin 

 coals. The following account gives the strata exposed on the 

 face of the quarry in 1867. 



