COAL UNDER SOUTH-EASTERN ENGLAND.
147
No sooner had this important theory, based on reasoning from observed facts,
been thoroughly started than the truth of the first part of it, that is the occurrence
of old rocks directly beneath the Cretaceous beds of the South-East of England,
was absolutely proved. This proof was given by the deep boring at Harwich,
which showed the occurrence, at a depth of about 1,015 feet below Ordnance
Datum,4 of Carboniferous rocks, but which are older than the Coal Measures.
At about the same time further evidence was given by the Kentish Town
boring, which reached red and grey beds of doubtful age, but unlike anything
Cretaceous or Jurassic, at about 930 feet below Ordnance Datum.
Other borings in and near London, and all made for water-supply, have con-
tinued the proof. In some cases these have shown more of the doubtful red
rocks, in others rocks of which the age is clearly marked by the contained fossils,
both Devonian and Silurian beds being found. There is no need to enter into
details of these deep borings, which have been fully described. Their sites are
Richmond, Streatham, and Crossness (on the south of the Thames), Kentish
Town, Meux's Brewery, Cheshunt, and Ware (on the north of the Thames).
Besides these, two other deep borings, at Chatham and at Dover Prison,
though they have not reached the older rocks, yet give evidence of the under-
ground thinning of the Lower Cretaceous and of the Upper Jurassic beds.
It should be noted that in some of these borings a thin representation of Lower
Jurassic beds comes between the Cretaceous and the older rocks, which last are
reached at depths of from about 710 to 1,222 feet below Ordnance Datum.
From what has been said above it is clear that the question of the presence of
a floor of Older Rocks next beneath the Cretaceous beds, or but slightly separated
therefrom by Jurassic beds, in South-Eastern England has for some years passed
out of the chrysalis stage of theory into the fully-developed stage of fact. A full
account of the question has been given in a Geological Survey Memoir ("The
Geology of London and of Part of the Thames Valley," vol. i., pp. 10-50, 1889),
to which the reader is referred.
Although the occurrence of a floor of Older Rocks at no enormous depth
beneath the surface was soon proved, geologists were still greatly at variance as to
the second part of the theory, the likelihood of the occurrence of Coal Measures.
There were not wanting some to contend that Coal Measures would not be
found underground in the South-East of England, but that the formation in ques-
tion was likely to thin out southward and westward from our known coal-fields.
Others, too, held that, should Coal Measures occur in such a position, they would
be found to be unprofitable as far as regards containing seams of coal of a work-
able character.
Others, however, who, it always seemed to me, had more reason on their side,
pointed to the fact that not only do Coal Measures occur at some depth under-
ground on the west, in the Bristol district, and on the east, in Belgium and
Northern France ; but that in both cases these hidden Coal Measures yield work-
able coal, which, indeed, was and is largely worked. This being the case, it was
argued that what had been proved to be the case at either end of the long line of
disturbance (from South Wales even to Western Germany) was likely to occur
also in the tract between, in which the range and extent of older rocks were little
known.
Strange to say, no attempt to prove the truth of either view was made in
England for many years, and all our knowledge was derived from borings made
4 In this and in the following cases the depth from the surface of course varies according to
the level of the site. The figures given are referred to one level, practically sea-level.