A HISTORY OF DURHAM 



times also the altered coal met with near intrusive dykes or sheets of 

 igneous rock is miscalled ' anthracite.' 



It is a kind of impure stony coal, useless for industrial purposes, and 

 locally known as 'cindered coal' (a good descriptive name), but it is in 

 no sense anthracite. The amount of ' ash ' or non-coaly mineral mat- 

 ter of the ordinary Durham coals is small in quantity — seldom indeed 

 more than the percentage of silica which the tissues of the coal-making 

 plants originally contained. In the cannel seams, especially towards 

 their outer limits (i.e. near the edges of the ancient ponds), the amount 

 of ash is often great, so much so that the cannels frequently pass later- 

 ally into shales (indurated and laminated mud). In the ' cindered coal ' 

 above referred to the percentage of ash is also very large, which would 

 not be the case were these metamorphosed coals akin to true anthracite. 



Before proceeding to enumerate the principal coal seams it will 

 be well to draw attention to the fact that the correlation of the seams 

 of one portion of the coalfield with those of another is often rendered 

 difficult by the frequent splitting up and reunion to which they are 

 subject. Mr. M. Walton Brown it was who first pointed out, by a 

 critical examination of all the evidence available a few years ago, how 

 all but universal is this division of the seams in the Great Northern 

 Coalfield. To this phenomenon, one which has not yet received a per- 

 fectly satisfactory explanation, it is largely due that the nomenclature 

 of the coal beds is so confusingly local and that there are so many 

 synonyms. 



Most of the seams to be now mentioned, in ascending order, are 

 under 6 feet in thickness and not less than 2 ft. 6 in. Thinner seams, 

 unless of some special interest, are omitted. 



Nos. I and 2 of the list are in the Lower Coal Measures, as above 

 defined, the rest are all in the so-called Middle and Upper Coal 

 Measures, divisions which, however convenient, are too empirical to be 

 recognized here. 



No. I. The Marshall Green Seam. — This coal lies only a little above 

 the Millstone Grit. It may be repeated that within the latter division 

 two or three thin and inconstant coals occur locally, but none of any 

 importance. 



No. 2. The Victoria Seam. — Known only in the western part of the 

 coalfield. 



No. 3. The Brockwell Seam, or Main Coal. — This is a coal of con- 

 siderable value and, as before stated, is generally taken as the Iiottom bed 

 of the workable Coal Measures (i.e. the so-called Mitldic and Upper 

 Coal Measures). The term Main is unfortunately also applied to other 

 seams. 



No. 4. The Three garter Seam. — Not to he confounded with 

 No. 10. 



No. 5. The Five garter Seam. — In some parts of the field this is 

 known as the Busty seam, in others as the IjOiver Busty. Not the same 

 as No. r 2. 



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