52 On Deneholes.
geological and geographical position. For while still deeper
Deneholes once existed—there can he little doubt—in the
neighbourhoods of Charlton Park and Blackheath, no shafts
are now open in those localities, the former existence of
Deneholes being only indicated by sudden subsidences such
as that which we attempted to investigate last year at
Blackheath. Want of funds obliged the Blackheath Sub-
sidences Committee to close their investigation before the
cause of these subsidences could be absolutely proved, but
the evidence obtained convinced most of us, sooner or later,
that the agency of Deneholes was—to say the least—the only
one that would bear any close examination. A chief obstacle
in the way of its acceptance was the fact that the excavators
of a Denehole at Blackheath would have had the water-
difficulty to contend with that caused the committee so much
trouble and expense—a difficulty which it was thought very
unlikely they would have been able to overcome. For at
Blackheath, above the Thanet sand and below the Blackheath
pebble-beds, lie about twenty feet of Woolwich beds, which
include ten to twelve feet of clayey strata. These clays hold
up the water which falls as rain on the pebble-beds at the
surface, and would of course have formed a hindrance to the
Denehole-makers not met with at Bexley or Grays. On the
other hand, however, this water-difficulty was absolutely fatal
to the opposite, or geological, theory of these subsidences.
For it showed that the rain which falls on the surface of
Blackheath never reaches the chalk at all, and consequently
can have no action upon it. Besides, we were operating with
a large shaft in previously shattered ground, whilst the
Denehole-makers worked with a small shaft in unbroken
ground. And as the water would be met with only in the
two or three feet of sand or gravel immediately above the
clayey shell-beds, it seemed probable that its percolation
into the shaft might have heen prevented by the simple
application of clay to its sides in sufficient quantity in that
small area.
Before the publication of Mr. Spurrell's paper, the Black-
heath Subsidences Committee was aware only of the existence