THE BRISTOL COALFIELD. 25 



ON THE GKITS AND SO-CALLED SANDSTONES OP 

 THE LOWEK AND MIDDLE SEKIES OF THE 

 BEISTOL COALFIELD. 



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



The Bristol Coalfield may be said to be noted for its series of Grits 

 and so-called Sandstones. These certainly have their equivalents in 

 the South Wales, Somersetshire, and Forest of Dean Coalfields ; and 

 possibly, if they could be distinguished, in other districts also. They 

 serve as stratigraphical land-marks, but owing to the difficulty in 

 distinguishing one from another, serious mistakes have arisen. 



The object of the present paper is (1) to compare these sandstones 

 with one another, with a view of ascertaining whether there are dis- 

 tinguishing features ; (2) to gain a more accurate account of the 

 general chemical composition;* and (3) to note the changes which 

 occur when they are in contact with carbonaceous matter. Their 

 relative positions will be seen on reference to the attached section of 

 the coalfield on page 26. 



First, I desire to draw attention to the application of terms grit and 

 sandstone. Rutley, m his " Study of the Rocks, "t gives the following 

 definition : — " These, when indurated, constitute sandstone, and when 

 more or less coarse grained and composed of angular and sub-angular 

 grains of sand are termed grits." In the first place, if angularity is to 

 define a grit, the term sandstone, as applied to beds of the Car- 

 boniferous period, appears to me to be an error, as in every case which 

 I have examined angularity has occurred. \ This feature is well shown 

 in the microscopic sections of the Pennant grit which are given, 

 and in those of the Millstone grit (see Plates II. and III. ;) but 

 in all that I have examined the grains of the latter show 

 the least angularity, and it is consequently least entitled to be 

 called grit, according to our present understanding of the term. It is 

 plain, therefore, that if angularity is to be considered in the matter, 

 grit must be more extensively applied to Carboniferous arenaceous rocks 

 than has been the case hitherto. Then as to coarseness : if that is to 

 be a guide to the application of the term grit, endless confusion will be 

 caused owing to the great variation iu one and the same deposit. In 

 the Millstone grit, for instance, and in that of the Pennant, (see 

 microscopic sections,) there is considerable variation, one bed is fine, 

 and another, a few feet apart, is coarse ; the two Pennant specimens 

 were taken from the same quarry. It therefore suggests itself to me that 

 it would be well to confine the term grit to those rocks the grains of 

 which are angular or sub-angular ; and to apply the term sandstone 

 where the grains of the rock are not angular, i.e., where the sharp 

 edges have been worn off. 



* And these, I take it, may be considered as typical of arenaceous Car- 

 boniferous rock generally. 



I Page 17. 



t Mr. Sorby was the firt=t to draw attention to this fact in his address to the 

 Geological Society of this year; but my observations were made quite apart 

 from his. I now, however, wish to bring them forward only as a confirmation. 



D 



