Oct., 1890. 
THE DOVER COALFIELD. 
233 
As long ago as 1840 attempts had been made in order to 
find the continuation of the coal measures beyond Douai, but 
the search was carried on too much to the west, towards 
Aires, and only resulted in establishing the fact that in that 
direction the coal was absent, Devonian rocks having been 
reached beneath the chalk and greensand. In the following 
year, in a boring for an artesian well at Oignies, shales, which 
appeared to resemble those of the coal measures, were struck, 
and in consequence the search for water was given up in 
favour of that for coal, and the result was that at a depth of 
about 1,400m. coal seams were actually pierced. Other ex¬ 
ploratory borings at Noeux (1845), as well as at other places, 
led to the sinking of pits, the first being made at Escarpelle, 
near Douai, and by 1855 the extent of the coalfield was pretty 
well ascertained. The workable beds of coal vary from 
0.50m. to 1.50m. At Lens there is a seam 2.20m. in thick¬ 
ness, and the average depth of the workings in this district is 
from 150m, to 250m., reaching to as much as 500m. at Ferfay. 
Previous to the discovery of the Pas ae Calais coalfield the 
coal measures had been worked in the Boulonnais, at Har- 
dinglien, in an upheaved portion of Carboniferous Limestone, 
and coal measures, underneath Cretaceous and Jurassic beds. 
Comparatively unimportant from an economical point of view, 
this small area is very interesting to the geologist, and indi¬ 
rectly to others, since it serves to show the lie of the beds, 
which are here considerably inverted. The sections of the 
Boulonnais coalfield show that the fragment of the Carboni¬ 
ferous beds which are here brought to light has been isolated 
by two faults parallel to its direction, and about 1,500m. 
apart, and it is doubtless a fragment detached from a larger 
mass which lies at a greater depth below the surface. It may 
even be connected with the Pas de Calais coalfield, as it is on 
the general line of the beds which run between Douai and 
Betliune. 
We are now in a position to follow these Continental 
measures across the Channel, and to judge of the line which 
they should take. The great ridge of Artois to the north of 
the Ardennes sinks gradually beneath newer deposits, and 
does not appear to have been so much disturbed there as in 
the exposed portion in the Ardennes. It can be traced at 
Brussels, where Silurian rocks appear at a depth of from 63m. 
to 122m., at Denderleew 151m., at Ninove and Grammont 
Palaeozoic rocks occur at 56m. and 46.75m. below the surface. 
Devonian beds are met with at Menin at 156.5m., Silurian at 
Flobecq at 57m. Again, at Tournai we find the Carboniferous 
Limestone 151m. deep, and finally Silurian strata atOstena at 
