THE BRISTOL COALFIELD. 59 



ON THE GRITS AND SO-CALLED SANDSTONES OF 

 THE LOWER AND MIDDLE SERIES OF THE 

 BRISTOL COALFIELD. 



BY EDWARD WETHERED, F.G.S., F.C.S. 



Continued from ptuj<' 29. 



The next deposit, then, is the Holmes grit, which is the base of the 

 middle series of the coal measures, and was for some time considered 

 to be the Millstone grit faulted up iu the southern part of the district. 

 By reference to the section of the Coalfield, the seriousness of such an 

 error becomes apparent, as the whole of the lower series of Coal 

 Measures would be unknown in that particular part of the district. It 

 was, too, considered to be identical with the Hard Venture grit. By 

 comparing the analyses of these various deposits, the similarity of 

 chemical composition renders the mistakes excusable if simply litho- 

 logical features are relied upon. The following are the analyses of 

 two specimens of typical Holmes grit, taken several feet apart : — 



Analysis or the Holmes Rock. 



No. 1. No. 2. 



Silica 9v80 9493 



Alumina 3 - 60 2-05 



Iron ; '70 -52 



Lime -40 -86 



Carbonaceous Matter "70 1 - 00 



Carbonic Acid . . '60 



Magnesia Trace. 



Water M4 -43 



10064 



Passing over one or two succeeding deposits of grit, marked in the 

 section of the Coalfield, we come to the principal rock of the district, 

 namely, that of the well-known Pennant grit, or, as it is generally 

 termed, Pennant sandstone. 



The Coalfield has usually been divided into three divisions, the 

 middle or Pennant grit series dividing the lower from the upper. It 

 would be almost better to leave out the term Pennant series, as it 

 gives rise to a mistaken notion as to the thickness of the actual 

 Pennant. On referring to the section of the Coalfield the construction 

 of the middle series is shown. It consists of several thick beds of grit, the 

 thickest of which is the Pennant, hence the name Pennant series. For 

 this reason the thickness of this grit has been placed, by some authors, 

 at 2,000 feet, the thickness of the entire middle series, whereas it is 

 only about 970 feot. 



I am unable to give the origin of the name Pennant, but the term 

 is applied to other beds in the district besides these just referred to. 

 This, I submit, is justifiable, if the term is to denote a particular clasa 



