A HISTORY OF DURHAM 



founded with the beds grouped under that name in the Scottish coal- 

 fields, which are equivalent to the Bernician Series) as usually accepted 

 may be defined as comprising the 200 or 300 feet of strata which 

 graduate upwards from the perfectly similar rocks of the Millstone 

 Grit, and come to an end immediately beneath the well-known lowest 

 continuous and valuable coal-seam known as the Brockivell or Main Seam, 

 which is regarded as the bottom bed of the so-called Middle Coal 

 Measures. 



These strata consist of sandstones, shales and a few sometimes work- 

 able but never quite constant coal-seams, together with ordinary fire- 

 clays accompanying such seams (or some of them), and a few beds, not 

 very continuous, of that hard white, compact, root-traversed and highly 

 silicious sandstone known as Gannister, and used for lining Bessemer 

 converters, etc. This singular rock is certainly more prevalent in these 

 beds than elsewhere in this region, but it is unfortunately by no 

 means restricted to them, as is the case in the Lower Coal Measures of 

 the Yorkshire and Lancashire coalfields for instance. Beds of the same 

 stone, sometimes quite as typical, are occasionally found in the Bernician 

 Series, where, here and there, they are even worked as Gannister, and 

 also in the higher Coal Measures, though to a less extent. Thus this 

 special deposit, though somewhat characteristic of the so-called Lower 

 Coal Measures (sufficiently so to justify the name Gannister Series, 

 sometimes applied to the division), can scarcely be used especially as it 

 occurs in non-continuous beds as a criterion of solid value. Again in 

 the more southern coalfields certain marine organisms of special types are 

 found which are restricted to some horizons in the Lower Coal Measures 

 and the Millstone Grit. This is not the case in Durham, though it is 

 possible, indeed probable, that further investigation may to some extent 

 put an end to this difficulty. This hope is held because in the adjoining 

 county of Northumberland casts of some of these fossils have been found 

 in these beds (in the neighbourhood of Stocksfield). More recently, in 

 shale cores from a deep bore in the Coal Measures in the north- 

 western portion of the Durham coalfield, from an horizon considerably 

 below that of the Brockwell seam, and either in the Lower Coal Mea- 

 sures or in the upper portion of the Millstone Grit, the writer detected 

 a small Productus^ a Discina and some annelid tubes allied to Serpulites. 

 These are of course marine fossils, but not specially of the kinds charac- 

 teristic of the Gannister Series of Yorkshire or Lancashire. 



The entire thickness of the Coal Measures is on the average some- 

 thing under 2,000 feet, but it must be remembered that denudation has 

 removed an unknown series of beds from the upper portion and that the 

 original thickness of the whole was certainly greater, and in all proba- 

 bility much greater than this. 



Just as in the Lower Carboniferous rocks the limestones are the 

 most persistent, and therefore, as datum lines, the most important beds, 

 so in the Coal Measures the thicker coal-seams are the deposits most to 

 be relied on in a survey of the strata. Insignificant individually as to 



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