366 
THE GEOLOGIST. 
crown many of tlie 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 " Bensham," 
*' 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 Eed 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 diflerence 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 Tyne, and the 
tender-coal, best suited for coking, is largely worked all along the line of 
the western outcrops from Pyton down to the outskirts of Paby 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 dykes. Of the others, 
the most noticeable is the great fault called the iVinety-fathom dyke, which 
starting from the coast near CuUercoats, where it displaces the strata to 
that amount, ranges past Gosforth to Blaydon, and then entering on the 
more hilly ground may be traced westward to the New Ped Sandstone of 
