412 
SILURIA. 
[Chap. XVII. 
group which has its full equivalent, as already shown, in Belgium and on the 
Rhine. Like the Lower Devonian of Brittany, the rocks in the Bas Boulonnais 
have a strike from the west-north-west to east-south-east, but are soon over- 
lapped, first by the Carboniferous, and then by the Cretaceous and younger 
deposits. 
Though the Devonian rocks of France are of no great vertical dimensions, and 
not comparable in thickness to the deposits of like age in Scotland, England, or 
the Rhenish Provinces, they are of considerable economic importance, particu- 
larly in certain schistose and arenaceous districts which they traverse, and where 
their limestone is much used in agriculture. At the extremity of the peninsula 
of Brittany these rocks appear in the Bay of Brest. 
Carboniferous Hocks of France. — In this work it is impossible to convey an 
adequate idea of the Carboniferous group, particularly that portion of it which 
is developed in all the coal-fields of France. The following few observations 
relate therefore only to the lower and calcareous member which immediately 
succeeds to the subjacent rocks, — the great Coal-fields of Valenciennes in the 
north, and those of St. Etienne and Autun in the south, being consequently 
unnoticed. In the Bas Boulonnais, as before observed, no gap or omission exists 
in this portion of the series ; for the Upper Devonian is at once followed by a 
thin patch of Carboniferous Limestone, with its large Producti. 
Mr. Godwin- Austen has dwelt upon the passage upwards of the Devonian 
rocks of the North of France into these Lower Carboniferous limestone and 
interstratified seams of coal, and has made an ingenious theoretical suggestion 
as to the probable extension westwards of the Coal-field of Valenciennes, so 
that a workable coal may possibly be found beneath the Tertiary and Cretaceous 
deposits of the London Basin and the south-east of England*. 
I took leave in the last edition of this work to dissent from this view, and 
have recently expressed the same opinion at the Nottingham Meeting of the 
British Association. My reasons for this unbelief are as follows : — In its exten- 
sion below the Cretaceous rocks towards the Straits of Dover, the Coal-field of 
Valenciennes thins out and deteriorates so much that to the west of Bethune it 
has become merely a narrow wedge. Next, in approaching the Channel this 
poor narrow zone is flanked to the north and the south by Devonian rocks lying 
at once beneath the Chalk (as proved by numerous borings), to the total ex- 
clusion of Coal-beds. These data have been all laid down in a ' Carte Indus- 
trielle du Bassin houiller du Nord de la France,' prepared upon the Government 
Map for the Compagnie de Vicogne, showing the numerous fruitless trials of 
speculators who obtained < concessions ' to search for coal, and only met with 
subjacent older limestones. With the exception, indeed, of the small patch of 
Carboniferous Limestone and a very little poor coal at Hardinghen, the Devonian 
limestones form exclusively the fundamental rocks of the Boulonnais, where 
they are covered at once by Jurassic or Cretaceous rocks forming the coast-cliff's. 
My belief therefore is, that, if the Secondary or Tertiary rocks on the English side 
of the Channel were pierced, the same rocks would in all probability be found 
as in the opposite part of France, and that either unproductive Carboniferous 
Limestone, or much more probably Devonian rocks would be met with. The 
boring for water undertaken at Harwich in the year 1857 has indeed demon- 
strated that the great productive Belgian coal-field has no extension in the 
East of England ; for after passing through 1030 feet of Tertiary and Creta- 
ceous rocks, a slaty impure limestone charged with Posidonomyae of the lowest 
* See Quart. Journ. Geol. Soc. Lond. vol. xii. p. 38 et seq. 
