366 



THE GEOLOGIST. 



crown many of the Fell tops are capped ; but before it bends downwards 

 to pass under the first strata of the coal-measures, we may frequently find 

 with it strata of shale and sandstone and fire-clay, roughly similar to those 

 of the true measures, but presenting to a practised eye peculiarities of 

 structure and colour. As we descend eastward from the higher ground of 

 the moorlands, on the edge of which the first Brockwell seam of coal is 

 traced, and as we find new and higher seams constantly succeeding, and the 

 strata inclined regularly towards the sea, we pass into the midst of that 

 tract which, extending from the river Coquet on the north to near the Tees 

 on the south, for 50 miles in length, forms the great northern coal-field. 

 The greatest thickness attained by this formation is probably not more 

 than 2000 feet ; but it would be vain for me, within a limited time, to offer 

 you details of the strata, — a subject which has been amply treated by Mr. 

 Buddie, Mr. Wood, Mr. T. Y. Hall, Mr. Greenwell, and others. Let it 

 suffice to say that in this thickness, there exist, associated with shales of 

 many varieties and with fine-grained sandstones, some fifty-seven beds of 

 coal, from an inch thick upwards, comprising in all 75 feet of coal ; but 

 that what are considered the workable seams are twelve in number, giving 

 an aggregate of about 50 feet of coal. The most famous of these seams, 

 from above downwards, are the high main, the yard coal, the "Benskam," 

 " five-quarter," " low main," " lower five-quarter," ; ' Euler or Button 

 seam," the "Towneley or Beaumont," the "Busty bank," " three-quarters," 

 and the " Brockwell." On the east the coal-measures are overlaid, in a 

 line running from South Shields past Houghton-le-Spring to near Bishop 

 Auckland, by the Permian series, represented by the magnesian lime- 

 stone and the Lower lied sand, that unequal and water-bearing bed which 

 forms the great obstacle to the sinking of shafts to the underlying coals. 

 Prejudice, it is well known, even after the difference of these strata from 

 the mountain limestone was proved, long contended that the coal would 

 not be found continuous beneath the magnesian limestone ; and it is still 

 asserted that the seams have proved inferior when they pass beneath it, as 

 shown especially by the failure, in certain tracts, of the Five-quarter and 

 Hutton seams. But no sufficient reason is apparent why such deterioration 

 is not rather to be ascribed to that variation in quality which all seams are 

 found to undergo when followed over a large area, than to the soil influence 

 of an unconformable upper formation. The variation here alluded to 

 exercises an important bearing on the commercial relations of different 

 parts of the field, and whilst the best " household coal " — bright, giving a 

 black cinder, and free from ash — extends from the Tyne to the Wear and 

 from the last river to Castle Eden, and occupies another area about Bishop 

 Auckland, the steam-coal, more dense and yielding a white ash, charac- 

 terizes the district beginning some five miles north of the Tj*ne, and the 

 tender-coal, best suited for coking, is largely worked all along the line of 

 the western outcrops from Eyton down to the outskirts of Eaby Park. 

 As regards the physical agencies which have impressed its present form 

 on this great coalfield, I would remark that they appear to have acted 

 with upheaval in a north and south direction, as evinced by the regular 

 strike over a great length of country. This was accompanied or followed 

 by transverse fractures resulting in several very pronounced lines of fault. 

 Two of these, running respectively E.N.E. and E.S.E., are the whin or 

 basaltic dykes named the Hett and the Cockfield d}~kes. Of the others, 

 the most noticeable is the great fault called the Ninety-fathom dyke, which 

 starting from the coast near Cullercoats, where it displaces the strata to 

 that amount, ranges past G-osforth to Blaydon, and then entering on the^ 

 more hilly ground may be traced westward to the New Red Sandstone of 



