52 
On Deneholes. 
geological and geographical position. For while still deeper 
Deneholes once existed—there can be 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 been 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 
